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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011)

Chapter: 2 Report of the Panel on the Galactic Neighborhood

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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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2
Report of the Panel on the Galactic Neighborhood

SUMMARY

The galactic neighborhood occupies a key role in our quest to understand the universe. Extending from the Milky Way and the Local Group out to redshifts z ≈ 0.1, the galactic neighborhood contains galaxies of all morphological types, metallicities, masses, histories, environments, and star-formation rates. However, unlike galaxies seen at greater distances, those within the galactic neighborhood can be probed with parsec-scale resolution down to faint luminosities. The resulting sensitivity permits the dissection of galaxies into their individual components, reaching the scale of individual stars and gas clouds. Moreover, these constituents can be studied in their proper context and with full knowledge of their galactic environment, allowing one to connect the stars and gas to the larger structure within which each formed. Thus, only in the galactic neighborhood can galaxies be studied as the complex, interconnected systems that they truly are, governed by microphysical processes. Probing this complexity involves studying processes that connect galaxies to extended gaseous systems: the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium (IGM).

The detailed observations possible in the galactic neighborhood also make it the critical laboratory for constraining the physics that governs the assembly and evolution of galaxies and their components across cosmic time. Indeed, almost every field of astrophysics—from the evolution of stars to the structure of dark matter halos, from the formation of supermassive black holes to the flows of gas in and out of galaxies—benefits from the detailed physical constraints that are

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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possible to achieve only in the galactic neighborhood. Not surprisingly, these constraints have been woven into the modern theoretical framework for galaxy formation and evolution.

To appreciate the impact of the galactic neighborhood, first consider studies of the universe on the largest scales. The interpretation of observations of the most distant galaxies is built on a foundation of knowledge established in the galactic neighborhood, including knowledge about the evolution of stellar populations, the existence of dark matter, the scaling relations of supermassive black holes, the effects of feedback from supernovae, the importance of accretion, the relationship between star formation and gas density, and the stellar initial mass function, among many others. Likewise, the evidence for dark energy from high-redshift supernovae was predicated on years of characterization of the properties of supernovae in nearby galaxies, along with more mundane constraints on the properties of dust extinction and exhaustive calibrations of the local distance scale.

The impact of the galactic neighborhood has been equally significant on smaller scales. The galaxies of the Local Group offer millions of observationally accessible stars, assembled into systems with a common distance and foreground extinction. The resulting samples of stars, their ancestors, and their descendants (e.g., planetary nebulae, supernova remnants, variable stars, transients, supernovae, molecular clouds, H II regions, X-ray binaries, etc.) can be analyzed with fewer uncertainties than in the Milky Way, where unknown distances and reddenings present challenging obstacles to assembling large samples. Moreover, such samples span a wide range of environment and metallicity, adding these new dimensions to the understanding of the physics of stellar evolution and the interstellar medium. The galactic neighborhood is also the only region where one can study the smallest scales of galaxy formation, revealing the presence of galaxies whose masses are scarcely more than a globular cluster. This fact is particularly important for assessing processes of feedback from star formation to the ISM, CGM, and IGM.

In assessing the scientific potential of the galactic neighborhood over the coming decade, the Panel on the Galactic Neighborhood faced a difficult task, given that the galactic neighborhood is the arena within which the interaction of nearly all astrophysical systems can be witnessed. Thus, narrowing down the scientific potential to only four key questions involved both the exclusion of research areas and unavoidable overlap with the scientific realms covered by other Science Frontiers Panels participating in the National Research Council’s (NRC’s) Astronomy and Astrophysics (Astro2010) Survey. This panel chose to focus its questions on areas in which the constraints from the galactic neighborhood are most powerful and unique. As a result, the four science questions developed by the panel exploit the use of the galactic neighborhood as a venue for studying interconnected astrophysical systems, for constraining complex physical processes, and for probing small scales. The key science questions are as follows:

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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  • What are the flows of matter and energy in the circumgalactic medium? This question concerns the understanding of the circumgalactic medium that is needed to understand the mass, energy, and chemical feedback cycle that appears to shape the growth of galaxies and the metal enrichment of the universe. In this report the panel identifies a program of detailed observations of the accretion and outflow processes in nearby galaxies that can inform the understanding of these processes at all epochs and mass scales.

  • What controls the mass-energy-chemical cycles within galaxies? This question explores the rich system of gas and stellar physics that shapes, and is shaped by, the interstellar medium. The panel outlines multiwavelength and theoretical studies of gas, dust, and magnetic fields within galaxies. Such studies can unravel the complexities of the gaseous ecosystem, with a level of detail critical to isolating the relevant physics but that cannot be obtained outside the galactic neighborhood.

  • What is the fossil record of galaxy assembly from first stars to present? This question focuses on probes of the fossil record of star formation, galaxy assembly, and the first stars. The panel identifies the value of surveys for resolved stars at high spatial resolution, with spectroscopic follow-up of stellar populations and metalpoor halo stars providing high-impact science unique to the galactic neighborhood. Furthermore, this fossil record promises to reveal the properties of galaxies at epochs where they cannot be seen directly.

  • What are the connections between dark and luminous matter? This question addresses the use of the galactic neighborhood as a laboratory of fundamental physics. The local universe offers the opportunity to isolate the nearest and smallest dark matter halos and to study astrophysically “dark” systems at high spatial resolution. The panel discusses the many observational and theoretical advances that could be expected as a result of these unique capabilities.

The prospects for advances in the coming decade are especially exciting in these four areas, particularly if supported by a comprehensive program of theory and numerical calculation, together with laboratory astrophysical measurements or calculations. The sections that follow this Summary describe the unresolved scientific issues in more detail, highlighting specific observational and theoretical programs that offer significant opportunities for advancing scientific understanding. Also highlighted is the discovery potential of time-domain astronomy and astrometry for capitalizing on powerful new techniques and facilities that provide precise connections among stars, galaxies, and newly discovered transient events.

Highlights of Top Activities Identified by the Panel

To make significant progress in addressing the four science questions, the panel suggests a broad program of ground-based and space-based science, together with

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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theoretical studies. In the highest overview, galactic neighborhood science uses the local universe as a laboratory for fundamental physics and astrophysics, galactic and dark matter structures, gas flows in and out of galaxies, and the fossil record of galaxy assembly. The astronomical goal is toward an understanding of how gas gets into galaxies, arranges itself to form stars, and returns to the galactic surroundings, reprocessed in the form of radiative, mechanical, and chemical “feedback.”

The science goals discussed in this panel’s report depend on the ability to trace the interconnected, multiphase nature of galaxies and their surroundings. This complexity naturally leads to a very broad range of desired observational and theoretical capabilities. Tables 2.1 through 2.4 at the end of this panel report summarize in some detail many of the possible capabilities that are mentioned in the sections following the Summary.

The panel recommends powerful new ultraviolet (UV) and X-ray missions for spectroscopic studies of these gaseous structures, chemical abundances, and flows. Studying processes within the galaxies requires capability at longer wavelengths (infrared [IR], submillimeter, millimeter, radio) to probe the processes that transform accreted gas into stars. Measuring the fossil record requires the identification of large numbers of stars through photometric and kinematic surveys and the subsequent study of their chemical content. Studies of star-formation histories through color-magnitude diagrams require both high spatial resolution on large optical and infrared (OIR) telescopes in space and high-resolution stellar spectroscopy on very large telescopes on Earth. Pursuing the connections between dark and luminous matter requires kinematic and abundance studies of dwarf galaxies and their stars, as well as of black holes that reside in many galactic nuclei, particularly in the Milky Way center. Progress in the areas of discovery potential identified by the panel can be made with new OIR and radio facilities that follow the transient universe and with powerful astrometric facilities.

GAN 1. WHAT ARE THE FLOWS OF MATTER AND ENERGY IN THE CIRCUMGALACTIC MEDIUM?

Observations over the past decade have revealed strong evidence for the infall and outflow of matter from galaxies. Less understood is the circulation of mass, energy, and chemical elements between galaxies and the IGM. This section discusses key issues of how gas gets into and out of galaxies—processes colloquially named “accretion” and “feedback,” respectively. The major feedback processes include the deposition and transport of mass, momentum, energy, and heavy elements into the ISM, CGM, and IGM by stars and supermassive black holes in galactic nuclei. The bulk of the energy and metals from these feedback channels is still not accounted for observationally. The common theme of these observations is emission-line and absorption-line spectra at moderate to high resolution.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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The current ideas of how gas gets into galaxies have emerged from cosmological simulations, with galactic mass as the key parameter. Galaxies with Mhalo ≥ 1012 solar mass accrete gas in the so-called hot mode after being heated to temperature (T) > 106 K by shocks near the virial radius. This shock does not develop in models of lower-mass galaxies, where gas flows along narrow filaments (“cold-mode” accretion), extending well inside the virial radius (Figure 2.1). This simple picture may provide a natural interpretation for the observed galaxy bimodality of stellar color and morphology, if one associates cold mode with blue, star-forming galaxies and hot mode with red, inactive galaxies. Firmly establishing such links to the stellar properties of galaxies requires observations that probe directly the gas-accretion modes.

Direct evidence for galactic outflows comes from the multispectral imaging of local galaxies (Figure 2.2) as well as the heavy elements detected throughout much of the IGM. Indirect evidence for gas flows comes from the observation that many galaxies contain lower fractions of baryons to dark matter than the primordial ratio, Ωbm. These missing baryons may reside in the IGM or in an extended multiphase CGM produced by interactions between galaxies and their intergalactic environment. Displacing these baryons to large spatial scales requires substantial energy deposition. Therefore, probing the content and circulation of matter, energy, and

FIGURE 2.1 Milky Way-size halo (left) at z = 2 fed by cold filamentary streams of gas (purple 104 K; yellow 106 K) into the virial radius. Milky Way-size halo at z = 0.1 (right) with infalling dense gas. SOURCE: M.E. Putman et al., How do galaxies accrete gas and form stars?, Astro2010 science white paper, submitted 2008.

FIGURE 2.1 Milky Way-size halo (left) at z = 2 fed by cold filamentary streams of gas (purple 104 K; yellow 106 K) into the virial radius. Milky Way-size halo at z = 0.1 (right) with infalling dense gas. SOURCE: M.E. Putman et al., How do galaxies accrete gas and form stars?, Astro2010 science white paper, submitted 2008.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 2.2 Messier 82 outflow—composite of Chandra, HST, and Spitzer images. X-ray data recorded by Chandra appears in blue; infrared light recorded by Spitzer appears in red; Hubble’s observations of hydrogen emission appear in orange; bluest visible light appears in yellow-green. SOURCE: X-ray: Courtesy of NASA/CXC/Johns Hopkins University/D. Strickland. Optical: Courtesy of NASA/ESA/STScI/AURA/The Hubble Heritage Team. IR: Courtesy of NASA/JPL-Caltech/University of Arizona/C. Engelbracht.

FIGURE 2.2 Messier 82 outflow—composite of Chandra, HST, and Spitzer images. X-ray data recorded by Chandra appears in blue; infrared light recorded by Spitzer appears in red; Hubble’s observations of hydrogen emission appear in orange; bluest visible light appears in yellow-green. SOURCE: X-ray: Courtesy of NASA/CXC/Johns Hopkins University/D. Strickland. Optical: Courtesy of NASA/ESA/STScI/AURA/The Hubble Heritage Team. IR: Courtesy of NASA/JPL-Caltech/University of Arizona/C. Engelbracht.

heavy elements between galaxies and the CGM is central to an understanding of galaxies and the IGM.

The high spatial resolution and panchromatic approach possible for nearby galaxies make the galactic neighborhood the best location for establishing the content and circulation of the circumgalactic gas. Indeed, observations from radio to X-ray have established the presence of galactic winds in starburst galaxies and feedback from active galactic nuclei (AGN) in massive elliptical galaxies. Optical observations that probe the CGM of distant galaxies sample a limited temperature range with little spatial information. A comprehensive understanding of galaxies and the IGM depends on an ability to probe the content and circulation of matter,

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

energy, and key elements. In the next decade, transformative gains in knowledge in this area will require two proven observational strategies: absorption-line tomography and spectral imaging. These techniques need to be applied to more targets at better velocity resolution so as to detect gas over a broad temperature range.

New observations at wavelengths most sensitive to these flows will help to remove uncertainties in simulations of galaxy formation and evolution, which currently lack the resolution required for the direct modeling of all physical processes. Physically motivated and empirically constrained recipes have been prescribed to model outflows in such simulations. But the amount of matter and energy in these outflows, the ultimate fate of the matter (escape or circulation), and their relation to the feedback process in more normal galaxies remain controversial. The coupling of the radiation, momentum, and kinetic energy from stars and supermassive black holes with the surrounding interstellar and circumgalactic gas is extremely difficult to model from first principles. Indeed, such models provide theoretical challenges for future simulations. Feedback mechanisms may enrich the IGM in heavy elements, create the mass-metallicity scaling relation for galaxies, suppress star formation in the smallest and largest galaxies, alter masses and shapes of galaxies over cosmic time, and connect bulge and black hole growth.

Distinguishing reality from plausibility will require well-articulated tests of the models. The observations must be sensitive to the physical and kinematic properties of the CGM, and they must be tested against higher-resolution simulations with additional physics.

What Is in the Circumgalactic Medium?

The circumgalactic medium is defined as the region around a galaxy that is strongly influenced by the galaxy’s gravity and by chemical and mechanical feedback. What are the overall mass, energy, and metal contents of the CGM? What are its spatial, thermal, chemical, and kinematic structures? These questions can be addressed in many wavelength bands, but most sensitively through absorption-line tomography of the CGM around nearby galaxies. For the Milky Way, the bulk of the CGM likely resides in a hot phase with T ~ 106-7 K, which can affect infalling clouds. The most sensitive way to detect and characterize this hot medium is absorption-line spectroscopy in the soft X-ray and UV regimes. These wavelength regions complement one another by probing different temperature regimes and ionization states in the CGM.

X-ray absorption-line spectroscopy is a powerful new tool for solving the missing-baryon and missing-feedback problems of galaxies, particularly for the Milky Way. This technique has been demonstrated in recent studies of the global hot gas in and around the Milky Way, based on the Chandra and X-ray Multi-Mirror Mission (XMM)-Newton X-ray grating observations of a few brightest

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

AGNs and X-ray binaries, typically with very long exposures. X-ray absorption lines produced by ions such as O VII, O VIII, Ne IX, and Fe XVII directly probe their column densities, which are proportional to the mass of the gas. Although individual lines are not resolved with the existing grating instruments, the velocity dispersion of the hot gas can often be inferred from the relative line saturation (e.g., by comparing O VII, Kα, and Kβ). These studies are leading to the first global characterization of the spatial, thermal, chemical, and kinematic properties of the ISM in our galaxy.

Requirements for X-Ray Absorption-Line Spectroscopy

Tomography of the CGM will require a substantially improved line detection sensitivity (, where R = λ/∆λ and A are the spectral resolution and photon collecting area, respectively) in the 0.3- to 1-keV range, which encloses the key metal lines. A resolution of 100 km s−1 (R = 3,000) and effective area A > 1,000 cm2 would provide 100 times improvement over Chandra and XMM-Newton, opening up the detection of metal lines from enriched outflows in the CGM and IGM. To conduct effective tomographic mapping, a few hundred sight lines are needed, which can be achieved by observing AGNs with fluxes greater than 10−11 ergs s−1 cm−2 for a reasonable exposure (for example, ~30 ks each), according to the source list from the Röntgen Satellite All-Sky Survey. The required line detection sensitivity is a factor of approximately 15 higher than that offered by the Chandra and XMM-Newton grating instruments. This sensitivity will allow one to use essentially all low-mass X-ray binaries in our galaxy and relatively bright ones in Local Group galaxies. The desired velocity resolution is ~100 km s−, which will produce line centroiding at 10 to 20 km s−1 (for S/N ≥ 10).

Tomography of the hot gas in and around the Milky Way will allow the decomposition of hot gaseous components of the Milky Way (galactic disk, bulge, halo) as well as of galaxies in the Local Group and local large-scale structure. Their global spatial, thermal, chemical, and kinematic structures can then be characterized. Absorption-line spectra toward the AGN sight lines can also be used to sample the CGM around intervening galaxies, at different impact parameters. With longer exposures, one can observe the fields of Local Group galaxies, and fainter AGN and binaries can be observed with longer exposures. The overall metal content and metal transport around galaxies can be inferred directly, allowing one to determine how these properties depend on the star-formation rate, mass, morphological type, and environment.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
How Are Galaxies Fed?
Understanding Cool and Warm Gas in the Circumgalactic Medium

An important issue related to the first key question is how the CGM controls the gas supply for star formation in galaxies. To address this question, both cool (T ≤ 104 K) and warm (T ≤ 105 K) gas in the CGM must be understood. This gas has been probed in 21-cm emission and a few optical lines (Na I, Ca II). However, by far the most sensitive probe is spectroscopy of the UV resonance absorption lines of metal ions and H I. The UV-optical wavebands include numerous diagnostic absorption lines from a wide range of ionization states of common elements from H to Zn, which provide measurements of gas temperature, density, metallicity, and kinematics in cold or cooling gas within the hot CGM seen in the X-ray. A great deal has already been learned about the cool CGM around the Milky Way, based on existing observations, particularly those from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph. In the next decade, the HST Cosmic Origins Spectrograph (COS) is expected to advance this technique significantly by applying it to potentially hundreds of galaxies in the galactic neighborhood. Obtaining these nearby external views will allow for a characterization of the projected distribution of cool gas around galaxies.

Requirements for Far-Ultraviolet Spectroscopy

The most desirable new facility for far-UV absorption-line spectroscopy of the CGM would be a follow-up of FUSE, but with a 10-fold increase in effective area. This capability will allow spectra to be taken of thousands of fainter AGN, as background targets for intervening galaxy halos. This facility will enable observations of O VI-bearing gas (not accessible in the COS pass-band) around many nearby galaxies, which are especially important for studying the interplay between hot and cool components of the CGM. Absorption studies can also probe the relationship between the hot CGM and infalling material, such as cool high-velocity gas clouds. Full three-dimensional information about the cool components of the CGM can be obtained from emission-line mapping: a wide-field spectrograph for Hα-emitting gas at approximately 104 K and 21-cm studies of H I clouds, using existing and upcoming optical and radio telescopes. Together with ever-improving numerical simulations (see, e.g., Figure 2.1), one can address questions such as the following: What is the origin of these clouds? How do they evolve in the CGM? Do they interact strongly with galactic disks?

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
How Does Galaxy Feedback Work?

Owing to the broad range of gas temperatures present in galactic flows, an accurate accounting of the outflowing material requires panchromatic, spectral-imaging observations (Figure 2.2). Such observations can identify the dominant physical mechanism driving the winds, as well as quantify the amount of mass, energy, and chemical feedback. Targeting galaxies out to redshift z ~ 0.05 would sample the full range of feedback activity, from dwarf and elliptical galaxies to extreme starbursts . This census would form the foundation for building a physical understanding of galaxy feedback.

The energy-carrying phase of a galactic wind emits primarily at X-ray frequencies, where observations have been limited to a small number of galaxies. Current facilities lack the velocity resolution required to measure gas kinematics in the hot phase, so only the thermal energy content (and not the kinetic energy) has been measured. Kinematic evidence for outflow lies with observations of much cooler gas, likely (but not definitively) entrained in the hot outflow. The relative speed of these components is model-dependent and not agreed on. Spectral imaging observations of the hot phase should be a key science driver for new X-ray instrumentation and missions.

Requirements for X-Ray Spectral Imaging

To observe a significant sample of perhaps 40 starbursts, the 0.3-10 keV sensitivity must reach fluxes of ~ 4 × 10−14 ergs s−1 cm−2. The imaging (5″ and 10-20′ field of view) needs to be sensitive to low surface brightnesses over kpc scales and to separate emission from hot plasmas and stellar sources, mainly bright X-ray binaries and supernova remnants, in galaxies at approximately 10 Mpc. The gas temperature may vary from 106 K to 108 K depending on the amount of mass-loading (mixing with cool, entrained gas). At the lower end of this temperature range, the plasma emission is dominated by X-ray line emission. Severe blending of these lines hinders the determination of the continuum level in existing spectral imaging data. Existing dispersive grating spectrometers are not designed for spatially extended targets and lack the sensitivity and spectral resolution required to study low surface-brightness emission. Direct measurement of the wind kinetic energy requires line centroiding to approximately 100 km s−1, achievable with full width at half maximum of 1,000 km s−1 and S/N ≥ 10 in the emission line. The required kinematic resolution would automatically enable the use of line-ratio-based temperature and ionization state diagnostics, greatly improving the knowledge of metallicity and thermal energy as well.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Requirements for Ultraviolet Spectral Imaging

Kinematic mapping of warm circumgalactic gas will provide insight into the interaction of a galaxy’s disk and halo. It will also reveal the circulation of matter between galaxies and cosmic filaments. Studies of the galactic halo suggest that warm gas constitutes a major component of the infall feeding the galaxy, but they are limited by distance uncertainties, which will not apply to observations of nearby galaxies. Emission-line imaging in the ultraviolet (e.g., O VI, H I Ly-α, C IV, C III, N V, O III, Mg II) will trace filaments and clouds to much larger galactocentric radii than is possible in the optical owing to the lower background. Inferring the electron density will provide the thermal pressure and absolute metal abundance of the CGM. Combining kinematic and line-ratio measurements from rest-frame UV spectroscopy distinguishes shock excitation, photoionization, and cooling radiation. Line ratios determine temperature and metallicity. When spatially mapped over the entire circumgalactic region, these diagnostics can distinguish infalling gas along filaments, clouds condensing and falling toward the galaxy, stripping of gas from merging subunits, outflows from galactic nuclei or star clusters, mixing of layers between phases with different temperatures, and shocks. For example, the collision of the hot wind with cool infalling gas produces radiative shocks, whereas mixing layers between the hot wind and entrained, interstellar gas radiate strong emission lines. The structural information gleaned from this approach would be nearly impossible to obtain from pencil-beam, line-of-sight studies, and it provides useful context for the interpretation of the measured physical information.

Imaging the CGM allows one to map gas topology and structure in all directions. The surface brightness of faint circumgalactic gas is typically 20 to 200 photons cm−2 s−1 sr−1. Emission from intergalactic gas will be fainter by an amount that varies widely among models. The 1,000- to 2,000-Å band-pass, particularly O VI, Ly-α, and C IV, is critical, and spatial resolution of a few arcseconds should be sufficient to remove point sources. A large field of view (20 arcmin) must be mapped to cover several tens of kpc in a large sample of local galaxies and allows campaigns to map several hundred kpc in selected environments. Spectral resolution must be sufficient to measure line-profile centroids to 30 to 100 km s−1 and probe feedback in the low-mass galaxies that appear to dominate intergalactic metal enrichment in the local universe. This spectral resolution may be realized by a FUSE-like spectrometer with a single aperture.

Requirement for Studying Neutral and Molecular Outflows

Cold winds from galactic nuclei may also be important for providing energy input to the IGM, for removing cold gas from massive galaxies, and for returning

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

heavy elements to the IGM. Evidence for cold winds has been revealed from the recent discovery of a fast molecular outflow from the S0 galaxy1 NGC 1266 and the observation of extraplanar dusty gas (e.g., see Figure 2.2). The importance of these neutral outflows to energize the IGM and clear their host galaxies of star-forming gas depends on how common they are, which is unknown at the present time. Observations with the Enhanced Very Large Array (EVLA), an expanded Allen Telescope Array (ATA), Combined Array for Research in Millimeter-wave Astronomy (CARMA), and Atacama Large Millimeter Array (ALMA), as well as Herschel, will be essential to developing an understanding of what drives these outflows without dissociating and ionizing the cold gas, and to allowing the measurement of how important these cold outflows are on global scales. For starburst galaxies, the above measurements will enable direct tests of the relative contributions of galactic superwinds, gas recycling, stellar-mass loss, and AGN feedback to determining the bulk properties of galaxies. For galactic disks, they test the galactic fountain model, while measurements of galaxies with substantial spheroidal components will provide new insights into the feedback from evolved stars and AGNs.

A full understanding of the multiscale and multiphase CGM requires a synergy of the approaches identified above, plus well-established observing and numerical simulation tools. Some specific questions to be addressed include the following: Are the properties of hot flows consistent with the thermalized energy from supernova explosions and stellar winds? Do cosmic rays and/or radiation pressure impart additional momentum to the low-ionization gas? What is the fate of Supernova Type Ia ejecta? Where is the kinetic and radiative energy from AGN deposited? What is the circulation timescale for ejected gas to be returned to the star-forming disk?

GAN 2. WHAT CONTROLS THE MASS-ENERGY-CHEMICAL CYCLES WITHIN GALAXIES?

How Do Galaxies Build Up Their Stellar Component Over Cosmic Time?

Important strides have been made in the past two decades in determining how galaxies grow from their dark matter seeds to the complex systems of dark matter, stars, and gas that are observed at z = 0. Simulations of the formation of structure in a universe dominated by cold dark matter (ΛCDM) have been able to reproduce the spatial distribution of galaxies and clusters of galaxies and have demonstrated the important role played by mergers. Over the same time period, observations of star-forming regions in the Milky Way have revealed some of the characteristics of the star-formation process, including both clustered and individual star formation

1

An “S0 galaxy” shows evidence of a thin disk and a bulge, but has no spiral arms and contains little or no gas.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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and the initial mass function. The challenge to astrophysics for the next decade is this: How do galaxies build up their stellar component over cosmic time?

To answer this question, the workings of the ISM on large and small scales needs to be understood. The story starts with intergalactic gas falling into the gravitational potential of a growing galaxy. Some key subquestions are these:

  • What are the signposts of the inflow?

  • How does gas become arranged within a galaxy?

  • How does some portion of this gas assemble into molecular clouds that provide sites for star formation?

  • How do conditions in and around these clouds determine the rate of star formation and the spectrum of initial stellar masses?

  • How does the energy released by stars affect the ISM and conversion of gas into stars?

  • How do stellar winds and supernova ejecta enrich the ISM with the heavy elements formed in stars and supernovae?

At the most basic level, there needs to be an understanding of the flow of baryonic matter and energy into galaxies (the accretion of intergalactic matter), within galaxies (secular evolution and flows into and out of stars), and sometimes out of galaxies by way of galactic winds, central jets, tidal interactions with other galaxies, and intergalactic ram pressure effects. Flows in the ISM within galaxies are driven by energy input from stars (radiation, high-velocity winds, supernovae), gravity, and infalling material (Figure 2.3). To understand the development of galaxies as stellar systems, the dynamics of the ISM must be understood on scales ranging from kiloparsecs (spiral structure), to parsecs (formation of giant molecular clouds, starbursts, and star clusters), and down to the sub-parsec level where star formation occurs. To understand the appearance of galaxies, there must also be an understanding of obscuration and scattering by dust and of emission by both dust and gas, all of which depend on the complex geometry of the ISM.

The evolution of galaxies at all redshifts depends on a detailed understanding of these physical processes and on knowing which ones dominate in different environments. However, studies of distant galaxies will always be limited by angular resolution. Understanding the dynamics and energy flows at earlier epochs will depend on having the most detailed observations and accompanying theory of nearby galaxies. Progress will require advances on multiple fronts: observations, theory, large-scale simulations, and laboratory astrophysics.

Star formation occurs primarily in clusters and associations, but there is almost nothing known about the conditions in the ISM that separate the production of bound clusters, OB associations, “super star clusters,” and globular clusters. Knowl-

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 2.3 Large-scale image of the Orion-Eridanus superbubble (~5,000 deg2) showing X-rays in blue (ROSAT), H II in red (WHAM), and H I in green as well as the brightest stars in Orion for scale. This image shows the importance of large-scale multiwavelength surveys for elucidating structures in the Milky Way interstellar medium. X-rays arise from hot gas heated by supernova explosions that sweep up cool H I. SOURCE: Courtesy of Carl Heiles, University of California, Berkeley.

FIGURE 2.3 Large-scale image of the Orion-Eridanus superbubble (~5,000 deg2) showing X-rays in blue (ROSAT), H II in red (WHAM), and H I in green as well as the brightest stars in Orion for scale. This image shows the importance of large-scale multiwavelength surveys for elucidating structures in the Milky Way interstellar medium. X-rays arise from hot gas heated by supernova explosions that sweep up cool H I. SOURCE: Courtesy of Carl Heiles, University of California, Berkeley.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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edge of these conditions is essential, not only for defining the physics of the modes of star formation but for connecting what is learned about star formation in the Milky Way and the nearest galaxies to star formation at higher redshift. After all, beyond z ≈ 0.1, individual stars are not observed, except possibly for O stars, and knowledge about them comes from observations of clusters and the integrated starlight of galaxies. The conditions that give rise to the stars and clusters that are observed in the nearby universe need to be extrapolated to those that exist at much greater distances.

In the next decade an increasingly clear picture of how stars form in the disk of the Milky Way will be obtained, but there is little understanding of how this relates to star formation in other galaxies. This is especially true for low-mass low-metallicity galaxies that are presumably the analogs of star-forming galaxies in the early universe. The discovery of ultrafaint dwarf galaxies in the Local Group challenges conventional wisdom about the formation of molecular clouds and star formation in such low-mass systems. It is unclear how molecular clouds form in systems with such weak gravity. Are their star-formation processes and histories similar to those observed in nearby low-mass galaxies? The puzzle of how stars form in close proximity to the black hole in the center of the Milky Way is an enigma whose solution can only be probed locally. A suite of millimeter- and submillimeter-wave single-dish telescopes and arrays, all equipped with array receivers, will be necessary to address these questions. Centimeter-wave arrays will also be needed to understand the flows of atomic gas into and out of galaxies; this gas is the raw material for the molecular gas that forms stars.

Star formation, at least in the Milky Way today, produces an initial mass function (IMF) that appears surprisingly insensitive to local conditions. Indeed, it has been difficult to find compelling evidence of variations in the IMF from one region to another. There is now preliminary evidence from nearby galaxies indicating that the IMF does vary, with an apparent deficiency of high-mass stars in outer disks and perhaps an excess of high-mass stars in galactic centers. In the coming decade, infrared and far-infrared imaging with Herschel and continuum observations with ALMA and future large-aperture single-dish submillimeter telescopes will be able to inventory any embedded high-mass stars missed by Hα observations. A 5- to 10-m aperture cooled far-infrared/submillimeter telescope in space would be a powerful tool for this purpose. It is important to carry out further studies to confirm IMF variations and to determine how the IMF depends on conditions in diverse star-forming regions. Understanding IMF variations as a function of environment at low redshift will inform the understanding of star formation at high redshift.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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The Multiphase Structure of the Interstellar Medium

The ISM spans a huge range of densities (~10−3 cm3 to >108 cm−3) and temperatures (10 K to 108 K) and includes magnetic fields and cosmic rays, both of which are dynamically important. The statistical properties of this fluid system remain poorly determined, including even the geometry and topology of the density field. Because the ISM is a chaotic system, large-scale numerical models can only be tested by comparing the predicted statistical properties (distribution functions for temperature, density, velocity; topological measures; anisotropic power spectrum of turbulence; and others) to those observed. The central question here is simple but sweeping: What are the structure and physical state of the ISM in the Milky Way and nearby star-forming galaxies? Only after these conditions in the ISM are known can there be hope of answering key questions such as the following:

What Controls the Radial and Vertical Transport of Mass and Metals?

The Milky Way and nearby galaxies are devouring their molecular star-forming gas at rates that are unsustainable for more than approximately 2 Gyr on average. Either the end of the star-forming era in the universe is near, which is unlikely, or the molecular gas lost to star formation is being replaced from reservoirs of H I in the outer parts of galaxies, or H II from the halo or IGM. The history of star formation and chemical evolution in the Milky Way disk appear to require a continuous delivery of low-metallicity gas from the CGM to the disk, but the process is not understood. The Milky Way’s high-velocity clouds are presumably part of this delivery system, but there may be other means by which gas is supplied to the disk. Gas in the outer parts of galaxies may be redistributed through tidal interactions in galaxy groups and/or radial inflows through the disks. Accompanying angular momentum transfer may also redistribute the raw material for molecular cloud formation. The transport of mass and metals out into the halo, and radially in the disk, is also important but poorly understood. Vertical transport will affect the metallicity of outflows from star-forming galaxies, whereas radial transport will alter the radial metallicity gradient in galaxies, which may not be entirely the result of local processing.

What Controls the Disk-Halo Interface?

There is no sharp boundary separating the gaseous disk from the gaseous halo and the flow of matter and energy between them. Superbubbles and “chimneys” resulting from correlated supernova explosions appear to be important for injecting gas, kinetic energy, and cosmic rays into the halo, as well as allowing O stars to photoionize gas far above and below the disk. This interface can be probed with

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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UV absorption-line spectroscopy and observations of diffuse infrared, optical, UV, and X-ray emission.

What Determines the Energetics and Star Formation of Far Outer Disks?

Several recent discoveries challenge the current understanding of the gas disks of spiral galaxies (Figure 2.4) that extend far beyond their bright optical disks. Atomic hydrogen has been found nearly 100 kpc from the centers of some galaxies, far more distant than had been previously known. Images from the Galaxy Evolution Explorer (GALEX) satellite have shown that star formation occurs, even in the most distant regions of H I disks, challenging present understanding of how and where molecules form and of how star formation takes place on global scales. Older issues such as how outer H I disks maintain nearly constant velocity dispersion with radius and what keeps gas disks thin in the apparent absence of stellar disks remain unanswered. Because most gas accretion likely takes place in the outer disks, understanding these phenomena appears to be critically linked to an understanding of galaxy evolution.

The Need for Multiwavelength Observations

Multiwavelength observations are required to characterize the ISM-IGM complexity.

  • Sensitive, high-resolution, all-sky 21-cm emission surveys out to z = 0.1 will be possible with the Allen Telescope Array if it is expanded to at least 128 and perhaps 256 dishes. The Expanded Very Large Array will be able to survey smaller portions of the sky in H I at yet-higher angular resolution, and the Green Bank Telescope (GBT) equipped with array receivers can survey the sky at more modest resolution, as can the Arecibo telescope with an upgrade to the number of 21-cm receivers that it currently has. These telescopes will be able to survey the Milky Way, galaxies out to moderate redshifts, as well as H I in the IGM. They will be able to determine the H I distribution and kinematics as well as the spin temperature on sight lines with background radio sources. The goal is to map ≥3 π steradians over the space of several years with 10 km s−1 velocity resolution and ~1,000 deg2 with 1 km s−1 spectral resolution, 1 arcmin spatial resolution, and 20,000 km s−1 velocity range. These facilities should be able to achieve σ(TA) = 0.5 K per pixel and have bandwidth sufficient to map ~1,000 deg2 to z = 0.5 in a few years of observation.

  • Millimeter- and submillimeter-wave observations of molecular lines and dust continuum in nearby galaxies using large single-dish telescopes and millimeter-wave arrays equipped with array receivers (in particular, CARMA) will be needed for mapping large areas at high resolution. ALMA will be incomparable for the high-

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 2.4 Three images of the grand-design spiral galaxy M51 at increasing wavelengths from left to right on the same scale. The collection of images shows that the spiral arms, seen in absorption in the visible, are seen in emission in the infrared and millimeter-wave radiation from the CO molecule. The far-infrared image also shows the thermal emission, tracing the distribution of dust heated in the galaxy, peaking in the spiral arms. SOURCE: Left to right: NASA/JPL-Caltech/R. Kennicutt (University of Arizona)/DSS; NASA/JPL-Caltech/R. Kennicutt (University of Arizona); ESA and the PACS Consortium; J. Koda, N. Scoville, T. Sawada, M.A. La Vigne, S.N. Vogel, A.E. Potts, J.M. Carpenter, S.A. Corder, M.C.H. Wright, S.M. White, B.A. Zauderer, et al., Dynamically driven evolution of the interstellar medium in M51, Astrophysical Journal Letters 700(2):L132, 2009, reproduced by permission of the AAS.

FIGURE 2.4 Three images of the grand-design spiral galaxy M51 at increasing wavelengths from left to right on the same scale. The collection of images shows that the spiral arms, seen in absorption in the visible, are seen in emission in the infrared and millimeter-wave radiation from the CO molecule. The far-infrared image also shows the thermal emission, tracing the distribution of dust heated in the galaxy, peaking in the spiral arms. SOURCE: Left to right: NASA/JPL-Caltech/R. Kennicutt (University of Arizona)/DSS; NASA/JPL-Caltech/R. Kennicutt (University of Arizona); ESA and the PACS Consortium; J. Koda, N. Scoville, T. Sawada, M.A. La Vigne, S.N. Vogel, A.E. Potts, J.M. Carpenter, S.A. Corder, M.C.H. Wright, S.M. White, B.A. Zauderer, et al., Dynamically driven evolution of the interstellar medium in M51, Astrophysical Journal Letters 700(2):L132, 2009, reproduced by permission of the AAS.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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resolution, high-sensitivity mapping of small areas and for high-molecular-line transitions and dust continuum. These telescopes will characterize the distribution and kinematics of molecular gas in structures including individual clouds, giant molecular clouds (GMCs), and dense gas in galactic centers that is likely fuel for energetic activity as well as the distribution of star-forming gas and dust in galaxies to z = 0.1. For high-resolution mapping, ALMA will be unsurpassed as long as the present plan to build at least 50 telescopes is maintained. However, the field of view is small, especially at higher frequencies (~5-25 arcsec). Therefore, array receivers and bolometric cameras with a large number of elements should be developed for deployment on CARMA, GBT, and the Large Millimeter Telescope (LMT) so that they may be ultimately installed on ALMA and future large single-dish facilities.

  • All-sky maps from Akari, Planck, and the Wide-field Infrared Survey Explorer (WISE) will provide new views of the distribution of thermally emitting dust on arc minute scales. As dust and gas are tightly coupled, the dust morphology reveals the distribution of the gas. Spitzer has surveyed star formation throughout the Milky Way and many external galaxies. Herschel (3.5-m aperture) will provide detailed images of selected regions, but a large (~20-m aperture) submillimeter telescope at a high-altitude site would allow high-resolution mapping at 350 μm. A 5- to 10-m cryogenic submillimeter telescope in space would permit dust continuum imaging with better resolution and much greater sensitivity than Herschel can provide.

  • High-resolution ultraviolet absorption-line spectroscopy remains the best method for determining the composition, ionization, and kinematics of interstellar gas (at T ≤ 3 × 105 K) along sight lines. Using background stars and AGN allows the three-dimensional structure of the gas to be delineated. The goal is to achieve ∆v = 1.5 km s−1 resolution, sensitivity down to λ = 1030 Å to study H2 absorption lines (λ < 1110 Å) and important ionic lines such as O VI 1032, 1038 Å, and the ability to achieve S/N > 100 on targets with mAB = 15.

  • High-resolution X-ray absorption-line studies can determine the distribution of abundant elements (such as C, O, Si, Fe) in different ionization states, as well as in solid grains. Such studies are especially important for determining the thermal, chemical, and kinematic properties of the hot gas in a galaxy. The goal is an X-ray telescope with line-detection sensitivity 10 times greater than that of Chandra or XMM-Newton. Energy resolution of ~1 eV would enable studies of grain mineralogy as well as gas kinematics.

  • Observations of diffuse ultraviolet and X-ray emission will permit high-temperature gas to be mapped and its temperature and ionization state to be diagnosed. Combined with absorption studies, this will determine the gas density and spatial scale of gas in various thermal phases. The required sensitivity and angular resolution are similar to requirements for measuring diffuse emission from the CGM or IGM. The diffuse Hα emission from the ISM is difficult to account

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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for with known sources of ionization (similar extended Hα emission is seen above and below the disks of other galaxies).

Some Enigmas

It is important to highlight several puzzling phenomena that appear incompatible with current models of the ISM and other galaxies. Because they may be indicative of serious problems with current models, the following phenomena should be pursued both observationally and theoretically: (1) the presence of very-small-scale neutral blobs in the ISM; (2) the so-called extreme scattering events that seem to indicate small, dense blobs of plasma in the ISM; and (3) molecular gas in almost 25 percent of E and S0 galaxies, in many of which no H I is detected.

What Is the Structure of the Magnetic Field in the Interstellar Medium?

Magnetic fields are dynamically important in the interstellar medium: strong enough to control gas motions in H I clouds and star-forming molecular clouds and to affect outflows from the disk into the CGM. Although present-day knowledge of magnetic fields in galaxies remains sparse, technological developments now permit observational progress:

  • Starlight polarimetry. Using visual wavelengths for diffuse regions and infrared (K band) for dense clouds, starlight polarimetry can provide detailed maps of the magnetic field within individual clouds. This capability will require the construction of imaging polarimeters for large telescopes at V, K, and possibly other wavelength bands.

  • Polarimetry of far-infrared/submillimeter emission from dust. Planck will provide all-sky maps of polarized submillimeter emission from dust on 5 arcmin scales in bright regions, and on ~1 deg scales from the average infrared cirrus. New instruments such as SCUBA-2—successor to the Submillimetre Common-User Bolometer Array (SCUBA)—and ALMA will be able to map the polarized emission from dust in molecular clouds and protostellar disks. Future instruments may permit far-infrared polarimetry from the Stratospheric Observatory for Infrared Astronomy (SOFIA). These maps, combined with other data, will disclose the three-dimensional structure of the magnetic field within a few hundred pc of the galactic plane and reveal the projected magnetic-field structure in nearby galaxies.

  • Zeeman effect. The Zeeman effect uses H I, OH, and CN to determine magnetic-field strengths in atomic and molecular gas. High-resolution all-sky surveys are now possible with radio interferometers. Such surveys require polarization purity of 20 dB and velocity resolution of 0.1 km s−1. The sensitivity and resolution are the same as given above for all-sky H I surveys.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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  • Faraday rotation. Faraday rotation uses pulsars and AGN to probe the line-of-sight component of the magnetic field weighted by the electron density. Ultrahigh-bandwidth instrumentation and radio telescopes, now being developed for all-sky Zeeman measurements, will be suitable for Faraday rotation studies.

  • Microwave synchrotron emission. The C-Band All Sky Survey will produce an all-sky map of polarized synchrotron emission near 5 GHz, where Faraday depolarization is modest, revealing the (projected) galactic magnetic field on ~1-deg scales.

Microphysics of the Interstellar Medium

To understand how galaxies form out of gas, it is necessary to improve the theoretical understanding of the important dynamical processes. Critical areas include the following:

  • Magnetohydrodynamics (MHD) and plasma physics theory. MHD simulations are increasing in sophistication and spatial resolution but still lack accurate representations of “subgrid” phenomena such as the decay of turbulence, magnetic reconnection, and ambipolar diffusion. Theoretical work must focus on these fundamental processes. Key questions are these: What processes are responsible for generating galactic magnetic fields? How does the field evolve?

  • Shock waves. Shock waves are ubiquitous in the interstellar and IGM, but there are still gaps in the theoretical understanding of phenomena including the following: the structure of collisionless shocks, cosmic-ray acceleration in shocks, magnetic-field amplification in fast shocks, coupling between Te and Ti in collisionless shocks, thermal conduction, multifluid MHD shocks in neutral clouds, and the role of charged dust grains.

  • Interstellar dust. Interstellar dust is important because of its role in attenuating and scattering light, its dynamical effects, and its value as a diagnostic tool (tracer of the gas, emission spectra sensitive to the local starlight intensity, and polarized emission and extinction sensitive to the local magnetic field). Using aligned dust as a tracer for magnetic fields requires an understanding of the shapes and optical properties of dust grains and of how variations in the degree of dust alignment depend on local conditions in clouds. The composition of dust varies within the Milky Way and between galaxies; scientists need to understand why. Observational studies, ranging from microwaves (emission from spinning dust) to X-rays (scattering and absorption by dust), provide a growing array of observational constraints which, together with advances in theory, will result in increasingly realistic grain models during the coming decade.

  • Atomic physics and laboratory astrophysics. Astrophysics is dependent on accurate wavelengths and oscillator strengths, photoionization and photodissocia-

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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tion cross sections, and rate coefficients for radiative recombination, dielectronic recombination, charge exchange, and collisional excitation. For example, there appears to be a factor-of-two uncertainty in the oscillator strength for the semiforbidden line, [C II] 2,325 Å, normally used to determine the gas-phase carbon abundance. Next-generation X-ray facilities, as well as ALMA millimeter-wave studies, will require more accurate wavelengths for lines, as well as more accurate X-ray absorption coefficients for likely astrophysical solids. Some quantities can be obtained from calculations, but others may only be obtained from laboratory measurements. It is reasonable to suspect that polycyclic aromatic hydrocarbons (PAHs) might account for the diffuse interstellar bands, but only careful measurement of PAH absorption cross sections in the gas phase in the laboratory can confirm and quantify this. Laboratory measurements are also needed for photoelectric yields from dust grains over a range of sizes, including PAHs.

As the “microphysics” are better understood and computational capabilities continue to increase, it can be anticipated that during the next decade numerical models will be able to account for the statistical properties of the ISM and star formation in the Milky Way and other galaxies. Only at this point will astronomers be able to claim to understand the formation and evolution of galaxies as stellar systems.

GAN 3. WHAT IS THE FOSSIL RECORD OF GALAXY ASSEMBLY FROM THE FIRST STARS TO THE PRESENT?

How galaxies form and evolve over cosmic time into the forms seen today has long been one of the most compelling of the big questions in astrophysics. Modern cosmology provides a theoretical paradigm for galaxy formation and assembly; astronomers can test these ideas with observations of galaxy properties such as morphology, luminosity, and color, and of the distribution of such properties in populations from nearby galaxies to redshifts that possibly overlap the epoch of reionization. As valuable and insightful as those observations will be, the study of ensembles necessarily obscures the physical processes affecting individual galaxies by providing only average properties. This is the frontier of galaxy formation studies: not only to determine the observed distribution of galaxy properties but also to explain how they came to be.

The galactic neighborhood can advance the understanding of galaxies with two essential complements to look-back studies. First, it is only in the galactic neighborhood that galaxies are fully resolved and open to intensive study at high spatial resolution. Second, the galactic neighborhood likely contains stars, and remnants of stars, that formed in early epochs and in small galaxies that will remain invisible at high redshift even in the next decade. This rich fossil record of galactic

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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star-formation histories is preserved in the colors of stellar populations and in the kinematics and abundances of individual stars. The basic astronomical techniques that allow the reading of this record of galaxy formation are now in place: the next decade promises to provide the tools needed to obtain and analyze this record in unprecedented detail and over a representative sample of the galactic neighborhood from the Milky Way to beyond 10 Mpc. In sum, these tools and techniques promise to reveal the full history of galaxy formation in galaxies of many different shapes and sizes, going all the way back to their origins in the first stars.

Fossil Record of Galaxies from Resolved Stellar Populations

The fossil record of whole external galaxies can be read in their stellar colormagnitude diagrams (CMDs) (see Figure 2.5). Progress in reconstructing the histories of local galaxies has been hindered primarily by the limited angular resolution available to resolve individual stars in nearby galaxies. From the ground, studies are limited to the nearest galaxies such as the Magellanic Clouds. Even there, limited angular resolution prevents one from determining the main sequence turnoff of the oldest population (which has the faintest turnoff) in the main bodies of these galaxies (where crowding is severe). The principal gains in the number of galaxies that can be studied in such a way came from the use of the Hubble Space Telescope, which opened up much of the Local Group, including the nearby dwarfs, the lowdensity regions of M31, and the galactic neighborhood out to 4 Mpc.

The variety in CMD morphologies implies a wide range of star-formation histories, which one can associate with such galaxy properties as gas content, environment, and morphology. One can even examine internal patterns, such as the relationship between stellar populations and spiral arms. With the HST, such work on a small number of galaxies promises a physical understanding of the processes that drive star formation in typical galaxies. However, current data rarely reach below the horizontal branch in these systems, do not include giant early-type galaxies (even the nearest ones lie more than 4 Mpc away), and are generally dominated by crowding errors even well above the completeness limits.

Progress in this field is currently limited by angular resolution and survey volume. With the HST, a general rule of thumb is that photometry of main-sequence stars can be done at surface brightness V ≈ 26 mag arcsec−2 until crowding sets in. This limits one to regions beyond 10 kpc along the minor axis or 25 kpc along the major axis of M31. Any gain in resolution will allow one to penetrate correspondingly further in. Analogously, to do comparable work in galaxies that are 10 times more distant would require a telescope with a diameter 10 times larger than that of the HST. Because the increase in aperture matches the decrease in flux from such stars, and because the increase in the background matches the decrease in pixel scale, the achieved S/N remains the same. Larger-aperture telescopes in space

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 2.5 Reconstruction of star-formation histories of M31. Top panels show radial velocity distributions (M31 systemic velocity is dotted line). Middle panels show stellar color-magnitude diagrams with superposed 47 Tuc fiducial. Bottom panels show reconstructed star formation history. SOURCE: T.M. Brown, R. Beaton, M. Chiba, H.C. Ferguson, K.M. Gilbert, P. Guhathakurta, M. Iye, J.S. Kalirai, A. Koch, Y. Komiyama, S.R. Majewski, et al., The extended star formation history of the Andromeda spheroid at 35 kpc on the minor axis, Astrophysical Journal Letters 685:L121, 2008, reproduced by permission of the AAS.

FIGURE 2.5 Reconstruction of star-formation histories of M31. Top panels show radial velocity distributions (M31 systemic velocity is dotted line). Middle panels show stellar color-magnitude diagrams with superposed 47 Tuc fiducial. Bottom panels show reconstructed star formation history. SOURCE: T.M. Brown, R. Beaton, M. Chiba, H.C. Ferguson, K.M. Gilbert, P. Guhathakurta, M. Iye, J.S. Kalirai, A. Koch, Y. Komiyama, S.R. Majewski, et al., The extended star formation history of the Andromeda spheroid at 35 kpc on the minor axis, Astrophysical Journal Letters 685:L121, 2008, reproduced by permission of the AAS.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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are crucial for obtaining the high angular resolution needed to do the science in crowded fields.

Second, let us examine the case for an increase in survey volume. The two principal drivers for increasing the volume are to probe a variety of environments and to obtain a representative mix of giant elliptical and S0 galaxies that make up much of the stellar mass in the local universe. Access to galaxies out to 10 to 20 Mpc will encompass the Virgo and Fornax clusters as well as numerous galaxy groups (Sculptor, M81, M101). The clusters include ellipticals, and the groups sample different morphological classes across the range of environments; this natural diversity in the galaxy population is inaccessible to the HST. An 8-m space telescope with diffraction-limited capabilities and high point spread function (PSF) stability in the UV and optical would be just able to probe this region, whereas a diffraction-limited 16-m telescope is needed to reach the main sequence effectively. A large space telescope would also dramatically reduce the time required to obtain star-formation histories of nearby galaxies, since the exposure time needed to obtain background-limited photometry on stars at a given distance declines as the fourth power of the aperture.

A number of approaches other than deep, visible-light CMDs can constrain the formation histories of galaxies over these volumes. One of these is to use the red giant branch (RGB) and asymptotic giant branch (AGB) stars as probes of the stellar populations of galaxies. These brighter regions of the Hertzsprung-Russell (H-R) diagram naturally extend the survey volume, but the requirements for angular resolution are still stringent. Adaptive optics (AO) offers dramatic gains in resolution in the near-infrared, where these bright cool stars are best detected, and current work on 8-m telescopes has extended these studies to galaxies at distances of several Mpc. While space telescopes with stable PSFs and low backgrounds are necessary for constraining the star-formation histories with the stellar main sequence at >10 Mpc, a 20-m ground-based telescope with diffraction-limited AO capabilities can reach 2 mag below the main-sequence turnoff in the Local Group, and a 30-m telescope may be able to reach the horizontal branch in the Virgo cluster at 16 Mpc. Stars in late stages of evolution are accessible in the Local Group (≤1 Mpc) with Spitzer and will be studied out to 10 Mpc with the James Webb Space Telescope (JWST). Matching star-formation histories obtained from UV-optical imaging with near-IR and mid-IR data will help unravel the nature of highly evolved stellar populations. The formation history of fair samples of both elliptical and spiral galaxies out to tens of Mpc can also be probed by other stellar population measures, such as globular clusters and spectroscopy of the integrated light of galaxies. These approaches utilize high-quality wide-field imaging and highly multiplexed deep spectroscopy, and they provide both stellar populations and kinematic tests of how individual galaxies in the local universe came to be.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Fossil Record of the Formation of Galactic Halo and Smallest Galaxies

Astronomical techniques for reading the fossil record of the Milky Way have come into their own in recent years in the form of large photometric and spectroscopic surveys. Because stars in the Milky Way can be examined for their individual kinematics and abundances in a way that stars in external galaxies cannot, it can be hoped that a deeper level of understanding can be reached regarding baryonic gas physics and chemical enrichment can be reached, even if the scope is narrowed to one galaxy. The Sloan Digital Sky Survey (SDSS) has made pioneering ventures into the discovery space that will be opened fully by future facilities. In conjunction with follow-up spectroscopy, the SDSS has shown that the Milky Way’s stellar halo is a complex structure that belies the traditional view of a smoothly varying density profile. Instead, the stellar halo is now known to possess at least two chemically and kinematically distinct spheroidal components, and within this is a rich array of substructure, including overlapping stellar streams and a dozen new ultrafaint dwarf satellite galaxies (Figure 2.6), some of which have a total luminosity comparable to that of a single giant star. These streams and small galaxies offer the opportunity to gain an understanding of the internal and external influences on galaxy formation at the smallest mass scale and with the most sensitive indicators of such processes as reionization, chemical enrichment, supernova feedback, and tidal disruption. A full unraveling of how gas collapses and forms stars down to these small scales and faint luminosities will illuminate the process of galaxy formation more generally. Lessons learned from local examples can then be applied to an understanding of the early epochs of galaxy formation when large galaxies such as the Milky Way of today were still composed of many such smaller components.

FIGURE 2.6 Detection limits for Local Group dwarf galaxies. The SDSS is only complete to ~50 kpc, while LSST could extend complete sample to 1 Mpc at Mv = −8. SOURCE: E.J. Tollerud, J.S. Bullock, L.E. Strigari, and B. Willman, Hundreds of Milky Way satellites? Luminosity bias in the satellite luminosity function, Astrophysical Journal 688:277, 2008, reproduced by permission of the AAS.

FIGURE 2.6 Detection limits for Local Group dwarf galaxies. The SDSS is only complete to ~50 kpc, while LSST could extend complete sample to 1 Mpc at Mv = −8. SOURCE: E.J. Tollerud, J.S. Bullock, L.E. Strigari, and B. Willman, Hundreds of Milky Way satellites? Luminosity bias in the satellite luminosity function, Astrophysical Journal 688:277, 2008, reproduced by permission of the AAS.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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When corrected for the difficulty of finding such faint objects at large distance, the observed counts of dwarfs imply that there may be hundreds of such dwarf galaxies waiting to be discovered in the Milky Way halo. If so, astronomers will have access to a significant sample of the smallest galaxies, many of which may carry stars formed during the epoch of reionization or even before. Deeper multiband all-sky surveys are needed to extend the search volume for such objects beyond the outer edge of the Milky Way halo and into the Local Group and to turn up additional streams and tidal features left over from the assembly of the Milky Way. At present, 8- to 10-m ground-based telescopes are used to obtain the radial velocities that confirm the structures found in the imaging surveys. The instruments needed to confirm detections by the forthcoming deep surveys will need to be correspondingly larger (20- to 30-m telescopes). Efficient multiobject spectrographs on such telescopes will be needed for measuring the chemical abundances for large samples of stars within these newly discovered galaxies, where 10-m-class telescopes can currently reach only a small handful in the closest faint dwarfs.

Fossil Record of First Stars and Galaxies from Galactic Archaeology

Ground-based observers recognized long ago that surviving remnants of the first stars and galaxies reside today in the Local Group, either directly through long-lived low-mass stars and white dwarfs, or indirectly through chemical abundances in the second-generation stars formed from gas enriched by the first heavy elements. The ability to survey and measure the properties of metal-poor stars throughout the Milky Way and Local Group may lead astronomers to the earliest epoch in the stellar fossil record—the first stars. These objects hold our interest as the ultimate beginning of galaxy formation and chemical enrichment in the universe. Theorists have reached the robust conclusion that the truly first stars formed in small dark matter halos at z = 10 to 30 and were likely massive (tens to hundreds of solar masses). Single stars at this epoch, even those with , are well beyond the reach of our current and forthcoming telescopes such as the JWST. Thus, if the theory is correct, it is certainly very difficult and perhaps impossible to test at high redshift.

A revolution is unfolding, thanks to large photometric and low-resolution spectroscopic surveys such as the Hamburg/European Southern Observatory (ESO) survey (HES) and the SDSS. Spectroscopic follow-up of these surveys may answer many of the open questions about the first stars and galaxies by surveying millions of metal-poor stars in the Milky Way and its dwarf satellites. Hundreds of thousands of metal-poor stars are already known, and hundreds have been subjected to high-resolution spectroscopic follow-up to measure abundances, often to 10 percent precision (Figure 2.7). This pursuit of the fossil record is distinguished from the previous two by a focus on the most metal-poor stars (below 1 percent

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 2.7 Abundance patterns for two hypermetal-poor stars in the galactic halo. Collecting complete abundance patterns with spectroscopy on large telescopes is a key observation. Interpreting these patterns with models of the first stars and chemical enrichment is a theoretical challenge. SOURCE: T.C. Beers and N.B. Christlieb, The discovery and analysis of very metal-poor stars in the galaxy, Annual Review of Astronomy and Astrophysics 43:531-580, 2005, copyright 2005 by Annual Reviews, Inc., reproduced with permission of Annual Reviews, Inc. in the format Other book via the Copyright Clearance Center.

FIGURE 2.7 Abundance patterns for two hypermetal-poor stars in the galactic halo. Collecting complete abundance patterns with spectroscopy on large telescopes is a key observation. Interpreting these patterns with models of the first stars and chemical enrichment is a theoretical challenge. SOURCE: T.C. Beers and N.B. Christlieb, The discovery and analysis of very metal-poor stars in the galaxy, Annual Review of Astronomy and Astrophysics 43:531-580, 2005, copyright 2005 by Annual Reviews, Inc., reproduced with permission of Annual Reviews, Inc. in the format Other book via the Copyright Clearance Center.

solar) and by the goal of obtaining precise abundances for elements from all of the important nucleosynthetic processes that act in stars, from which much information can be obtained about the population of stars that produced the metals. This effort has raised many questions, all ripe for progress in the next decade: How old are the oldest stars in the Milky Way? Where are the lowest-metallicity stars in the Milky Way and when did they form? What were the IMF and chemical yields of the first stars? Did the IMF vary with metallicity and galactic conditions, even after the first stars? When did the IMF become normal and universal? Where are the heavy elements created (particularly neutron-capture elements)? Can chemical tagging of metal-poor stars be used to identify coeval populations, later dispersed around the galaxy?

The first step in the process will be realized by the same large photometric and spectroscopic surveys that map the Milky Way. New data-mining algorithms must be developed to select efficiently the most metal-poor stars from these vast samples, and efficient multiplexed spectrographs on large telescopes will be needed to obtain abundances of the elements that tell with statistical confidence the full story of primordial star formation and chemical enrichment. This field also poses a stiff challenge to theory that must synthesize realistic models of the Milky Way over the full 13- to 14-Gyr history of the universe with mass dynamic range of over 107, and calculate chemical evolution in the full abundance space available to modern stellar surveys. This field has a strong link to nuclear astrophysics; supernova yields derived with nuclear physics inputs are crucial for chemical evolution calculations, and the most metal-poor stars promise in turn to reveal the production sites and abundance ratios of the heavy chemical elements and perhaps lead to the discovery of novel supernova mechanisms. Finally, all of the work on resolved stellar populations in the Milky Way could be greatly enhanced by precise astrometric observations for large samples of stars. These measurements could provide precise

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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orbits (see below, the subsection titled “GAN D2. Astrometry as a General Area of Discovery Potential”). This benefit will be enhanced further if future astrometric missions and/or facilities focus their attention on the same regions of the galaxy to be covered by the stellar populations surveys.

In the next decade, the high-redshift frontier of direct look-back studies of galaxies should be advanced beyond z > 10 by the JWST, ALMA, and perhaps by some high-z 21-cm experiments. However, theory predicts that there should still be galaxies and stars beyond that frontier. The enormous potential of the fossil record to address this epoch in a complementary way will likely be realized in the next decade, when the synthesis of high- and low-redshift data promise to bring a full understanding of how the stellar components of galaxies are formed and assembled back to the earliest epochs at which stars were formed.

GAN 4. WHAT ARE THE CONNECTIONS BETWEEN DARK AND LUMINOUS MATTER?

Dark matter is the dominant constituent of mass in the universe, but it has only been detected indirectly. Scientists have inferred its existence dynamically in the local universe, and through the perturbations that it has imprinted on the early universe. However, a detailed characterization of the nature of dark matter has defied years of effort, beyond a consensus that it behaves nonrelativistically on large scales and has no significant nongravitational interactions with normal baryonic matter. Here, the panel outlines the importance of the local universe with respect to making progress in one of the greatest unsolved mysteries of modern astrophysics.

Using the Local Universe as a Dark Matter Laboratory

The lambda cold dark matter (ΛCDM) paradigm has had success on a range of astrophysical scales. On the largest scales (>10 Mpc), it can fit the spatial clustering of galaxies, the distribution of temperature anisotropies in the cosmic microwave background (CMB), and the clustering of hydrogen absorption in the Ly-α forest. On smaller scales (0.1-10 Mpc), the density profiles of galaxy halos and cluster dark matter predicted by numerical simulations appear to agree with those inferred from gravitational lensing and galaxy kinematics. On the smallest scales, however, ΛCDM has not yet been verified. These scales offer some of the most sensitive probes of the properties of the dark matter, owing to the close linkage between the innermost structure of collapsed dark matter halos and the high initial phase-space density that characterizes current ΛCDM models. At these scales, there is a well-known tension between the high central densities and cusped profiles predicted by ΛCDM within the centers of galaxies, and the somewhat lower densities and flatter profiles that kinematic observations favor on sub-kiloparsec scales. Furthermore,

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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ΛCDM predicts a large abundance of very small dark matter halos, but to date this substructure remains largely undetected. Whether or not these conflicts require modifications of ΛCDM is a subject of some debate. Although models exist that provide potentially adequate solutions with baryonic physics, these are neither demonstrably correct nor unique.

Complicating these comparisons is the complex interplay between dark matter and baryons within galaxies. The global distribution of baryons within a galaxy is, to first order, controlled by the mass and accretion history of the dark matter. However, the final structure of the dark matter can be strongly affected by the behavior of the baryons, through bars, gravitational contraction, and outflows. Moreover, the baryons can frequently obscure the underlying properties of the dark matter, such as in cases where the baryons dominate the mass in the inner regions of a galaxy.

The effects of baryons must therefore be disentangled before any modification of ΛCDM can be considered truly imperative. As is discussed below, likely approaches include the following: identifying systems where baryons are negligible, constraining the amount of low-mass substructure, improving the understanding of inner halo kinematics, and directly detecting signals from dark matter interactions at high densities. These experiments will inevitably focus on the galactic neighborhood, which is the only environment in which astronomers have sufficient sensitivity and spatial resolution to conduct such tests, including those related to the following questions:

  • What is the distribution of dwarf satellite galaxies in the Milky Way? Is it consistent with the predictions of the standard ΛCDM cosmological model, and what does it tell about the relation between the stellar and dark matter substructure?

  • What is the distribution of dark matter in the Milky Way? What are the relative densities of baryonic and dark matter, and are they consistent with the standard cosmological model?

What Is the Baryon-Dark Matter Connection at Low Galaxy Masses?

Because lower-mass galaxies appear to be increasingly dominated by dark matter, their kinematics are excellent probes of the dark matter potential. One must find systems of the lowest-possible luminosities and measure their internal kinematics, both radial velocities and proper motions. The first step relies on finding overdensities of stars in deep multicolor photometric surveys and confirming the overdensities with spectroscopy. The SDSS has made substantial progress, primarily within the Milky Way’s virial radius. Dramatic increases in identified dwarfs will come with deeper imaging and even-wider-area surveys. The second step requires spectroscopic follow-up of red giant branch stars (0 > MI > −4) ith better than

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 2.8 Mass within 300 pc versus luminosity for the classical (blue dots) and “ultrafaint” (red dots) dwarf galaxies. Remarkably, while the luminosity varies by more than four orders of magnitude in the observations, the mass only varies by factors of order unity in the central region and only 1-2 orders of magnitude for “total” halo mass. SOURCE: Reprinted by permission from Macmillan Publishers Ltd: Nature, L.E. Strigari, J.S. Bullock, M. Kaplinghat, J.D. Simon, M. Geha, B. Willman, and M.G. Walker, A common mass scale for satellite galaxies of the Milky Way, Nature 454:1096-1097, 2008, copyright 2008.

FIGURE 2.8 Mass within 300 pc versus luminosity for the classical (blue dots) and “ultrafaint” (red dots) dwarf galaxies. Remarkably, while the luminosity varies by more than four orders of magnitude in the observations, the mass only varies by factors of order unity in the central region and only 1-2 orders of magnitude for “total” halo mass. SOURCE: Reprinted by permission from Macmillan Publishers Ltd: Nature, L.E. Strigari, J.S. Bullock, M. Kaplinghat, J.D. Simon, M. Geha, B. Willman, and M.G. Walker, A common mass scale for satellite galaxies of the Milky Way, Nature 454:1096-1097, 2008, copyright 2008.

~1 km s−1 resolution, currently possible with 8-m-class telescopes at the distances of the ultrafaint dwarfs found with SDSS (Figure 2.8).

Measuring kinematics becomes challenging for the faintest galaxies, with total luminosities less than that of a single star at the tip of the red giant branch. For such systems, kinematics must be measured for stars of even lower absolute magnitudes than for typical dwarfs with fully populated color-magnitude diagrams.

The baryons in ultrafaint dwarfs are also signposts of processes at the extremes of galaxy environments. Their shallow gravitational potential wells make them susceptible to disruption by feedback from galactic winds, suppression by reionization, and stripping by outflows from nearby massive galaxies. Hints of this fragility can be seen in the variation in mass-to-light ratio of the faintest dwarf galaxies, which span four orders of magnitude in luminosity but have comparable velocity dispersions. The mass limits below which galaxies cannot retain baryons or form stars are currently not known for certain. More-sensitive multiobject spectroscopic or astrometric facilities would open the possibility of measuring hundreds of velocities per galaxy. If extensive spectroscopy is coupled to astrometric measurements in future surveys (~10 μ arcsec), the three-dimensional velocity structure would eliminate uncertainties with respect to the galaxy masses at the limits of galaxy

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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formation. Higher signal-to-noise spectroscopy (S/N > 20 at R ≥ 20,000) could constrain the ages and abundance patterns.

How Much Low-Mass Substructure Exists Locally?

ΛCDM predicts an order of magnitude more low-mass dark halos than can be accounted for by the current local inventory of small galaxies (classical and ultrafaint dwarfs). This well-known “missing satellites” problem has two possible solutions. The first appeals to baryonic physics, which can suppress the formation of luminous dwarfs leaving behind a population of dark halos, lacking detectable baryons. The second solution requires the modification of the ΛCDM paradigm itself, which would be necessary if the first solution proves untenable. If more prosaic baryonic physics is responsible, then every ultrafaint dwarf should be the “tip of the iceberg” for a much larger population of invisible dark matter halos. This population is most likely to be detected indirectly, through their gravitational influence on luminous matter: gravitational lensing of background objects, kinematic distortions in apparently isolated galaxies, excess kinematic heating of dwarfs, or seeding of gas accumulations in tidal tails.

Can the Properties of Particle Dark Matter Be Constrained Directly?

If the dark matter is a weakly interacting particle, then it should annihilate with some small probability, producing gamma-ray and/or X-ray radiation in currently favored models. The galactic center is probably the brightest source in the sky for such annihilations, but the astrophysical backgrounds in this region make it difficult to disentangle photons produced by the candidate dark matter particle from those produced by supernova remnants, pulsars, and binaries. The best place to look for the signature of weakly interacting dark matter may be in the heart of the ultrafaint dwarf galaxies. With proportionally fewer stars for their central masses, the astrophysical backgrounds are lower, yet they have high central densities, increasing the rates for any n2 emission process. Figure 2.9 shows the likelihood for gamma-ray annihilation of a dark matter particle within the Constrained Minimally Super-Symmetric Model; these predicted fluxes are below the current sensitivity of the Fermi Telescope. Convincing detections of emission from dark matter would give the first conclusive maps of the inner dark matter density profiles.

Galactic neighborhood science will also help interpret results from direct detection experiments, which are slowly narrowing the possible regimes in which a weakly interacting massive particle (WIMP) dark matter particle can exist. These experiments should prove interesting over the coming decade. The particle physics community has a growing interest in the distribution and kinematics of dark matter within the Milky Way at the solar circle. Such constraints are likely to come

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 2.9 Contours of likelihood of predicted E > 1 Gev gamma-ray flux versus the dark matter particle cross section, using current data from the dwarf galaxies SEGUE-I (left panel) and Draco (right panel). The inner and outer contours represent the 68 percent and 95 percent confidence regions, with the color and scale bar representing significance normalized to the best-fitted value. These fluxes are below the sensitivity of Fermi. SOURCE: G.D. Martinez, J.S. Bullock, M. Kaplinghat, L.E. Strigari, and R. Trotta, Indirect dark matter detection from dwarf satellites: Joint expectations from astrophysics and supersymmetry, Journal of Cosmology and Astroparticle Physics 2009:014, 2009.

FIGURE 2.9 Contours of likelihood of predicted E > 1 Gev gamma-ray flux versus the dark matter particle cross section, using current data from the dwarf galaxies SEGUE-I (left panel) and Draco (right panel). The inner and outer contours represent the 68 percent and 95 percent confidence regions, with the color and scale bar representing significance normalized to the best-fitted value. These fluxes are below the sensitivity of Fermi. SOURCE: G.D. Martinez, J.S. Bullock, M. Kaplinghat, L.E. Strigari, and R. Trotta, Indirect dark matter detection from dwarf satellites: Joint expectations from astrophysics and supersymmetry, Journal of Cosmology and Astroparticle Physics 2009:014, 2009.

from a combination of astrometric and kinematic constraints on the Milky Way potential (see below, the subsection titled “GAN D2. Astrometry as a General Area of Discovery Potential”) and numerical simulations of the gravitational focusing of dark matter by baryonic disks.

What Are the Mass Contribution and Kinematics of Baryons at Higher Galaxy Masses?

Although dwarf galaxies are superb laboratories for probing dark matter physics, they are only one extreme of the halo mass distribution. It is therefore critical to push the constraints on dark matter structure to higher masses. Doing so requires coping with the population of baryons that dominates the inner galaxy. The current requirements for doing so are (1) two-dimensional kinematic maps of the gas and stars, with arcsecond resolution; (2) accurate constraints on the mass contributed by the stellar and gaseous components, including molecular and hot gas; (3) fully self-consistent kinematic modeling to include the effects of noncircular motions; (4) characterization of the mass distributions at very large radii, using tracers such

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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as globular clusters, distant stars, satellite galaxies, or hot gaseous halos; and (5) constraints on the three-dimensional shape of the halo, using kinematic tracers reaching above the major axis of the galaxy. There are a handful of systems for which more than two of these constraints currently exist.

The first three of these required capabilities can be reached with existing 4-to 8-m facilities, but they require much larger investments of observing time. In contrast, measuring kinematics at large radii and/or off-plane is only marginally feasible with current technology, and it usually requires statistical co-adding of the few available tracers (globular clusters, satellites, sparse stellar clusters, etc). Progress on requirements (4) and (5) above would be accelerated by the addition of spatially resolved X-ray spectroscopy of surrounding X-ray emitting gas with kpc spatial resolution and 100 km s−1 spectral resolution, or deployable integral field units on 8-m-class telescopes with arcminute fields of view. Within the Milky Way, three-dimensional large-radius kinematics could be obtained using the velocities and positions of hypervelocity stars, halo RR Lyrae, and blue horizontal branch stars, all of which can be identified in large surveys and followed up astrometrically and spectroscopically. Constraints on the three-dimensional structure of galaxy halos and the full angular momentum distribution of baryons both offer new tests of the predictions of ΛCDM.

A Census of Fundamental Parameters for Black Holes

Massive black holes play an increasingly important position in astronomy and astrophysics. On small scales, their immediate environments probe the extremes of gravitational potentials. On larger scales, massive black holes in galactic nuclei may play major roles in the evolution of galaxies. Over the next decade significant progress is expected in constraining the distribution of the two fundamental properties of black holes—mass and spin—and their relationship to their host galaxies. This progress will come primarily from studies of nearby galaxies and the Milky Way, the only systems that can be observed with the necessary spatial resolution and sensitivity.

What Controls the Masses of Black Holes?

It is now widely accepted that most galaxies host black holes in their nuclei and that the masses of the central black holes tend to be higher in more massive galaxies. However, this simple picture leaves open the basic question, Why? It is currently not known how the central black hole forms, what processes couple the final mass of the black hole to that of the surrounding galaxy, and why some galaxies appear not to have nuclear black holes, or at least appear to have masses below the current sensitivity limits. It is generally accepted that supermassive black holes gain mass

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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from accretion, but the exact process of accretion is far from understood. One of the key questions for understanding the growth of central black holes in galaxies is their initial mass (the seed mass). Models include massive black holes starting as ~102 stellar remnants, as “intermediate-mass” black holes, or as already massive black holes with masses of and above. Although some evidence suggests that intermediate-mass black hole seeds grow to become supermassive black holes, detections remain sparse and controversial. The local universe offers the most promising avenues for identifying potential seed black holes, by direct detection through dynamical studies of nearby systems, by indirect studies of low-luminosity AGNs, and through the possibility of measuring gravitational waves of black hole inspiral events. Advances in spatial resolution (AO systems) and sensitivity (larger telescopes) will enhance the most-used techniques for measuring black hole masses. Detection of gravitational waves will open up a new avenue for characterizing the demographics of black holes.

The relationship between the mass of the supermassive black hole at the center of a massive galaxy and the surrounding spheroid of stars is one of the most significant discoveries of the past decade. Most current research focuses on the relationship with velocity dispersion or bulge luminosity. These relations have become important benchmarks for theories of galaxy and quasar growth, and it is thus critical to pin down how the masses of central black holes and their host galaxies are related. The local universe provides the anchor for these correlations, with unique capabilities for determining the parameters (slope, normalization, dispersion) of the trends, in much the same way that the Tully-Fisher and Faber-Jackson relationships have been used as diagnostics for galaxy formation as a whole.

Although current studies have opened up this field and provided the basis for a very broad range of work, they are incomplete in a number of key ways. For example, the sample size with highly accurate data is modest, the trends and scatter at low and high stellar masses and dispersion are not well established, and potential systematics in the analysis remain unconstrained. In the next decade, advances in the determination of the black hole relationships will come from a number of areas, including improved spatial resolution using stellar dynamics (from AO-equipped 8-m-class telescopes), more accurate gas kinematics (from AO and improved radio observations of maser disks), improved constraints on kinematics at large radii (requiring longer integrations on 8-m-class telescopes), and more detailed modeling with existing computing facilities (using triaxial and N-body models, and including dark halos).

Finally, significant questions remain at the limits of the nuclear black hole mass function: At the one extreme, what limits the growth of the most massive central black holes? At the other, why do some galaxies appear to have no central black hole at all, even when they host dense nuclear stellar clusters? Is this lack of detection simply a sensitivity issue?

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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What Is the Distribution of Black Hole Spins?

Understanding the physics of accretion around massive black holes has been a struggle ever since quasars were first discovered. One possible diagnostic of the accretion process is the final spin of the black hole. Observational evidence generally supports the idea that black holes are near maximal spin, although current results are subject to varying interpretations. If true, this result will have significant consequences both for the transfer of angular momentum between the accreting material and the black hole and for the merger histories of black holes. Significant improvements in measures of black hole spin over a wide mass range are possible over the next decade, coming primarily from improved X-ray observations with large collecting areas and good spectral resolution (∆E < 3 eV at 6 keV) and from variability measures at long wavelengths. Theoretical work related to black hole accretion, such as the proper treatment of magnetohydrodynamics, is also advancing significantly, and the combination of sophisticated numerical modeling with local measures of spin and flux can provide significant advances in the understanding of accretion disk physics.

The Galactic Center and Sgr A*

The black hole at the center of the Milky Way galaxy is a remarkable object, providing astronomers’ best opportunity to observe the environment around a massive black hole and its interaction with the surrounding galaxy. The central parsec of the galaxy demonstrably contains young stars in ordered motion, with a stellar cusp around the central object. These observations have challenged expectations for star formation near a black hole. The existence of young stars in the vicinity of a black hole is hard to understand, given the high stellar density and activity level; this has fueled an active debate as to whether they are formed in situ or fall in from outer radii. The M31 nucleus also shows evidence for very young stars around a black hole, possibly in a disk. Although the Milky Way black hole provides unprecedented detail, other nearby systems need to be explored so that the generality of the trends can be understood. Continued studies in the Milky Way will quantify the range of stellar ages, their orbits, and the properties of the central density cusp (stellar and, potentially, nonbaryonic). These results will reveal how these stars form and how the black hole influences star formation at the galactic center, both of which questions have general implications for the formation of galaxies and stellar associations.

Additional scientific possibilities in the study of the central black hole in the Milky Way would become possible with significant improvements in depth and spatial resolution. These advances would increase the number of stars that could

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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be studied and enable more detailed orbit determinations for stars with smaller pericenters. Broadly speaking, as astronomers measure stars that approach closer to the black hole, they expect to see stronger general relativistic effects and to produce tighter constraints on the central mass distribution (the black hole mass and central cusp). Specific advances that may be enabled include limits on, or the discovery of, a binary black hole, constraints on mechanisms for producing hypervelocity stars, and the possible detection of an event horizon. The needed improvements in spatial resolution and sensitivity could come from high-precision astrometry on systems with higher Strehl ratios (e.g., extreme AO systems) and/or larger-aperture telescopes than are currently available, including improvements in long-wavelength imaging (e.g., submillimeter VLBI images). While important first theoretical steps have been made, truly quantitative predictions require a combination of models of black hole growth that span accretion disk scales as well as galactic scales.

DISCOVERY AREAS

GAN D1. Time-Domain Astronomy as a Galactic Neighborhood Area of Discovery Potential

The panel views the exploration of the transient sky, where enormous swaths of parameter space remain essentially virgin territory, as a potent area of discovery. New areas of parameter space have always led to new discoveries, and there is every reason to think that examining the sky on timescales from nanoseconds to years across the entire electromagnetic spectrum will lead to significant scientific discoveries and new insights. Moreover, the availability of new instruments, along with ever-increasing computational capability and algorithm development, makes the transient sky an area particularly ripe for transforming basic understanding regarding the content of the galactic neighborhood.

The galactic neighborhood is essential for characterizing and interpreting transient phenomena. Measuring the distance, energetics, and demographics of newly observed phenomena is the first essential step in understanding the underlying physics. Transient events observed in nearby galaxies allow one to characterize the luminosities of the events, their rates, and connections with underlying stellar populations and galactic structure with an ease that is not possible in the galaxy where distances are hard to measure, or over cosmological distances, at which high-resolution imaging and spectroscopic follow-up is difficult. Obtaining follow-up spectra is important to enable time-domain studies to reach their full scientific potential.

Time-domain studies of the galactic neighborhood promise a wealth of information on the properties of supernovae, variable stars, late-stage mass loss from

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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evolving stars, binary stars, the disruption of stars near the Schwarzschild radius of central black holes, the flickering of central engines, as well as unanticipated phenomena. Time-domain studies can also address the nature of some unexplained phenomena, such as variable galactic radio sources that have no obvious counterparts at other wavelengths and so-called extreme scattering events seen in the radio. Even well-understood, time-variable stellar phenomena such as RR Lyrae, Cepheids, and long-period variables offer unambiguous tracers of the galactic neighborhood’s ancient stellar populations, thereby giving a three-dimensional structure of stellar streams and their orbits, and thus constraining our galaxy’s dark matter halo.

In summary, time-domain astronomy within z < 0.1 will allow scientists to map the content and evolution of stellar structure in galaxies at a level of detail and precision not easily obtained outside the local universe. It will also secure the base of the distance ladder, providing constraints on H0 that can take us to a new level of cosmological precision. Insights offered by an expansive view of the local transient sky are likely to lead to a deeper understanding of the structure of dark matter and the values of cosmological parameters.

GAN D2. Astrometry as a General Area of Discovery Potential

Astrometry can open a new window for the discovery of extrasolar planets; discover and characterize vast numbers of Kuiper belt objects, asteroids, and comets; test the weak-field limit of general relativity with unprecedented precision; and measure the aberration of quasars from the centripetal acceleration of the Sun by the galaxy. These surveys can provide a complete inventory of stars near the Sun, with accurate masses for a wide range of stars, particularly for rare objects at the extremes of the Hertzsprung-Russell diagram. They can measure orbits of the globular clusters and satellite galaxies of the Milky Way and galaxies of the Local Group and fix properties of the major stellar components of the Milky Way. The most important tools are large-scale photometric, spectroscopic, and astrometric surveys. Prototypes in past decades were the Two Micron All Sky Survey (2MASS), the SDSS, the Infrared Astronomy Satellite (IRAS), and Hipparcos. Larger, deeper, more accurate surveys have exceptional discovery potential in the next decade, largely from the variety of powerful astrometric techniques now reaching maturity:

  • Radio astrometry of masers in massive star-forming regions yields accuracies of a few microarcseconds (μas). These measurements yield the following: (1) accurate (to a few percent) distances and velocities for approximately 20 objects several kiloparsecs away, (2) estimates of the distance to the galactic center and the rotation speed of the local standard of rest that are arguably the most accurate available, and (3) the first proper motions of galaxies other than the Milky Way and its

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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satellites. Radio astrometry of maser sources in AGN accretion disks provides the best masses for black holes at the centers of galaxies, with the possibility of many more, and offers the prospect of determining the extragalactic distance scale (the Hubble constant) with unmatched precision.

  • Radio astrometry of the source Sgr A*, believed to coincide with the black hole at the galactic center, now yields the most accurate measurement of the angular speed of the Sun in its galactic orbit, as well as strong constraints on the mass of the black hole and the mass and orbit of any possible companion black hole(s). Infrared astrometry of the stars around Sgr A* proves that it really is a black hole rather than a compact stellar cluster and gives its mass and distance at steadily growing precision.

  • Space-based optical astrometry is capable of achieving astrometric accuracies of a few micro arcseconds on hundreds of target stars or ~20 μas for more than 10 million stars, as will be done with the Gaia mission of the European Space Agency.

  • Time-resolved ground-based astrometry using large optical surveys can provide proper motions and photometric parallaxes for millions of stars; and can identify unusual high-proper-motion objects that may be faint nearby stars and hypervelocity stars.

SUMMARY OF DESIRED CAPABILITIES

Note that the specifications listed in Tables 2.1 through 2.4 are only approximations of the type of capabilities that would drive galactic neighborhood science in the next decade. This panel was not charged with making detailed analyses of technical capabilities and defers to the Program Prioritization Panels in this regard (see Part II of this volume).

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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TABLE 2.1 General Capabilities: Theory, Astrometry, and Time-Domain Imaging and Spectroscopy

Capability

Specifications

Scientific Applications

Theory

Support for theory must accompany all major new facilities

Simulations of IGM, CGM, galaxy formation

Modeling galaxy feedback

Supermassive black hole spin and accretion

Microphysics of accretion

Connection to large-scale galaxy formation

Astrometry

σπ ~ 1-10 μas

Mapping the Milky Way

Determining fundamental parameters of the Milky Way

Determining stellar-mass spectrum

Tests of general relativity around Sgr A*

Precision measurement (1-3%) of H0

Time-domain imaging

Multiple visits, N ~ 100

Cepheid/RR Lyrae studies in Local Group

Optical transients, radio transient sky (almost totally unexplored)

Time-domain spectroscopy

R ~ 5,000 resolution

Black hole masses

NOTE: Acronyms are defined in Appendix C.

TABLE 2.2 Capabilities: Short-Wavelength Bands (UV, X-Ray, Gamma-Ray)

Capability

Specifications

Scientific Applications

Gamma-ray spectroscopy

F ~ 10−13 cm−2 s−1 (broadband, 10% resolution)

Dwarf galaxies and the dark matter particle

X-ray imaging spectroscopy

0.3-8 keV at R ≈ 300

FOV ~ 20′ and ∆θ ~ 5″

Ilim ~ 10−14 ergs s−1 cm−2 arcmin−2

Mapping hot gas outflows, supernova remnants, superbubbles

0.5-7 keV at 1-3 eV resolution

Mapping hot plasma in SNRs, stellar winds, superbubbles

Line detection sensitivity ~ 10× Chandra 0.5-7 keV

Black hole mass measurements

Dark matter indirect detection

X-ray absorption spectroscopy

0.1-1 keV

R ≈ 3,000

Aeff ~ 1,000 cm2

Abs-line spectroscopy of hot CGM

Abs-line spectroscopy of hot ISM

Dust grain mineralogy

Inventory of abundant elements in ISM; obtaining kinematics of gas

UV imaging

1,000-3,000 Å

R ≈ 1,000-2,000

FOV ~ 20′ and ∆θ ~ 1″

Ilim ~ 100 LU (ph s−1 cm−2 sr−1)

Now: STIS, GALEX

Needed: ∆θ ~ 1″

Mapping warm ISM and CGM

Mapping galaxy outflows

Mapping hot plasma in the ISM and at the disk-halo interface

Mapping star-forming regions

UV spectroscopy

900-3,000 Å at R ≈ 30,000

Aeff ~ 104 cm2

Now: STIS and COS (1,150-3,000 Å)

Abs-line spectra of warm ISM, CGM

Diagnose hot plasma in ISM and at disk-halo interface

Tomography of ISM, CGM

Composition and kinematics of both hot and cold gas, excitation of H2

Need: <1,030-3,000 Å

R = 30,000-100,000 to mlim ~ 19

NOTE: Acronyms are defined in Appendix C.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 2.3 Capabilities: Optical and Infrared Bands

Capability

Specifications

Scientific Applications

Optical polarimetry

B, V, R, I imaging polarimeter on large telescope

p < 0.05% for mAB = 18

Mapping magnetic field in diffuse ISM

IR polarimetry

J, H, K imaging polarimeter ∆p < 0.05% for mAB (K) = 18

Mapping magnetic field in dense clouds

UV/optical/IR imaging

0.01-0.1″ spatial resolution (HST successor)

V ~ 35 or 0.5 mag below main-sequence turnoff Two-color photometry

5-10′ FOV

Stable PSF and low backgrounds

CMD-based star-formation histories for galactic neighborhood galaxies (Q3.1, Q4)

 

Substantial sky coverage (SDSS successor)

V ~ 18-28

 

 

Multiband photometry

∆θ = 0.04″

mlim ~ 25

FOV ~ 10′

Locating and identifying faint Milky Way halo substructure (Q3.2, Q4) including streams and dwarf galaxies

 

∆θ = 0.04″

mlim ~ 21

Probing Milky Way black hole

Proper motions of Milky Way stars

Optical/near-IR spectroscopy

R = 10,000-40,000, V ~ 18-19

500-1,000 multiobject spectrograph 105-106 stars

Kinematics and abundances in Milky Way halo, dwarfs, and streams (Q3.2); power is in sample sizes, and so efficient multiplexing matters

 

R = 40,000, V ~ 18-20

Precise (0.1 dex) relative abundances in individual EMP stars (Q3.3) to assess star-formation history and IMF in early galaxies

 

R = 2,000 to V ~ 24

10′ FOV, 100 multiobject spectrograph

Kinematics of resolved populations in external galaxy halos (Q3.1) important to separate overlapping spatial components

 

R = 400-2,000

V ~ 18

Large FOV (~all-sky coverage)

Obtain relative abundances to moderate precision

Efficient preselection of EMP star candidates (Q3.3) based on spectroscopically measured bulk metallicity

NIR spectroscopy

R ≈ 3,500

∆θ = 0.04″

1-3 μm

Black hole mass measurements

NOTE: Acronyms are defined in Appendix C.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 2.4 Capabilities: Long Wavelengths (IR, Far-IR, Submillimeter, Radio)

Capability

Specifications

Scientific Applications

Far-IR imaging and spectroscopy

Now: Herschel (3.5-m warm telescope)

Mapping dust distribution in nearby galaxies with ~5″ resolution

 

Needed: 5- to 10-m cold telescope

Study of major far-IR lines (63 μm [O I], 158 μm [C II], 205 μm [N II] )

 

2× better angular resolution

>103× greater sensitivity

 

Submillimeter imaging and spectroscopy

Now: ALMA (bands 7-9) for high resolution

Needed: ~25-m telescope at high site

∆θ < 3″ at 350 μm

350 μm-1 mm

Polarimetry (linear) would be valuable

High-resolution line and continuum images of star-forming regions’ high-excitation molecules, dust

Mapping star-forming clouds at galactic center

Dust maps for nearby galaxies

Potential for imaging black holes

Millimeter-wave imaging

Approved: ALMA

High-resolution images of dust in star-forming regions

 

Needed: CARMA array receivers (1 mm)

High-resolution images: molecular gas in star-forming regions

Connecting galactic to extragalactic star formation

Millimeter-wave line and continuum mapping

Now: ALMA in S, CARMA in N

Distribution of CO and other molecules in star-forming galaxies

 

Needed: CARMA array receivers (1 mm)

Needed: LMT large-format array receivers

Zeeman measures of B|| from CN

Molecular-cloud properties in different galactic environments

Centimeter-wave line and continuum mapping

Now: EVLA, GBT, Arecibo, ATA

Needed: upgrade Arecibo and GBT to array receivers

Needed: upgrade ATA to 128 antennas

Improved H I maps, deep extragalactic imaging

Zeeman measurement of B|| using H I, OH

Mapping synchrotron and free-free emission

Identifying and mapping new heavy molecules and transitions

Gravity waves

Detection

Black hole masses

NOTE: Acronyms are defined in Appendix C.

Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
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Suggested Citation:"2 Report of the Panel on the Galactic Neighborhood." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics Get This Book
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Every 10 years the National Research Council releases a survey of astronomy and astrophysics outlining priorities for the coming decade. The most recent survey, titled New Worlds, New Horizons in Astronomy and Astrophysics, provides overall priorities and recommendations for the field as a whole based on a broad and comprehensive examination of scientific opportunities, infrastructure, and organization in a national and international context.

Panel Reports--New Worlds, New Horizons in Astronomy and Astrophysics is a collection of reports, each of which addresses a key sub-area of the field, prepared by specialists in that subarea, and each of which played an important role in setting overall priorities for the field. The collection, published in a single volume, includes the reports of the following panels:

  • Cosmology and Fundamental Physics
  • Galaxies Across Cosmic Time
  • The Galactic Neighborhood
  • Stars and Stellar Evolution
  • Planetary Systems and Star Formation
  • Electromagnetic Observations from Space
  • Optical and Infrared Astronomy from the Ground
  • Particle Astrophysics and Gravitation
  • Radio, Millimeter, and Submillimeter Astronomy from the Ground

The Committee for a Decadal Survey of Astronomy and Astrophysics synthesized these reports in the preparation of its prioritized recommendations for the field as a whole. These reports provide additional depth and detail in each of their respective areas. Taken together, they form an essential companion volume to New Worlds, New Horizons: A Decadal Survey of Astronomy and Astrophysics. The book of panel reports will be useful to managers of programs of research in the field of astronomy and astrophysics, the Congressional committees with jurisdiction over the agencies supporting this research, the scientific community, and the public.

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