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



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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 result- ing 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 53

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Panel rePorts—new worlds, new HorIzons 54 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 superno- vae 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 small- est 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 com- ing 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 po- tential 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:

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rePort Panel GalactIc neIGHborHood 55 of tHe on tHe • 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 stud- ies 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 metal- poor halo stars providing high-impact science unique to the galactic neighbor- hood. 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 small- est 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 understand- ing. 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

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Panel rePorts—new worlds, new HorIzons 56 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 (in- frared [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 subse- quent 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, en- ergy, 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.

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rePort Panel GalactIc neIGHborHood 57 of tHe on tHe The current ideas of how gas gets into galaxies have emerged from cosmologi- cal simulations, with galactic mass as the key parameter. Galaxies with Mhalo ≥ 1012 solar mass (M◉) accrete gas in the so-called hot mode after being heated to tem- perature (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, Wb/Wm. These missing baryons may reside in the IGM or in an extended multiphase CGM produced by interactions between galaxies and their intergalactic environ- ment. 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 10 4 K; yellow 106 2-1.pdf 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.

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Panel rePorts—new worlds, new HorIzons 58 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,

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rePort Panel GalactIc neIGHborHood 59 of tHe on tHe energy, and key elements. In the next decade, transformative gains in knowledge in this area will require two proven observational strategies: absorption-line tomog- raphy 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 cur- rently 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 cou- pling 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 galax- ies 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 proper- ties 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 feed- back. 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 infall- ing 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

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Panel rePorts—new worlds, new HorIzons 60 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 veloc- ity dispersion of the hot gas can often be inferred from the relative line saturation (e.g., by comparing O VII, Ka, and Kb). 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 (S/N ∝ RA , where R = λ/Dλ 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 (R∝OSAT) 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−1, 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 decom- position 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.

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rePort Panel GalactIc neIGHborHood 61 of tHe on tHe 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 exter- nal 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 obser- vations 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 re- lationship 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 Ha-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?

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Panel rePorts—new worlds, new HorIzons 62 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 (star-formation rates > 100 M◉ yr−1). 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 frequen- cies, 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 instru- mentation and missions. Requirements for X-Ray Spectral Imaging To observe a significant sample of perhaps 40 starbursts, the 0.3-10 keV sensi- tivity 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 spec- tral 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 re- quired 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.

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rePort Panel GalactIc neIGHborHood 63 of tHe on tHe 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-a, 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 radi- ation. 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 direc- tions. The surface brightness of faint circumgalactic gas is typically 20 to 200 pho- tons 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-a, 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 resolu- tion 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

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Panel rePorts—new worlds, new HorIzons 84 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 pro- saic 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 influ- ence 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 cen- tral 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 de- tection 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 phys- ics 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

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rePort Panel GalactIc neIGHborHood 85 of tHe on tHe 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 phys- ics, 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

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Panel rePorts—new worlds, new HorIzons 86 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 prop- erties 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

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rePort Panel GalactIc neIGHborHood 87 of tHe on tHe 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 104 M◉ “intermediate-mass” black holes, or as already massive black holes with masses of 105 M◉ and above. Although some evidence sug- gests 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 cen- ter 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 ex- ample, 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?

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Panel rePorts—new worlds, new HorIzons 88 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 gener- ally 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 (DE < 3 eV at 6 keV) and from variability measures at long wavelengths. Theoretical work related to black hole ac- cretion, 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 expec- tations 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

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rePort Panel GalactIc neIGHborHood 89 of tHe on tHe 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 discov- ery. 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 underly- ing 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 infor- mation on the properties of supernovae, variable stars, late-stage mass loss from

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Panel rePorts—new worlds, new HorIzons 90 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 unex- plained 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 Ly- rae, 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 com- ets; 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 accu- racies of a few microarcseconds (mas). 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 avail- able, and (3) the first proper motions of galaxies other than the Milky Way and its

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rePort Panel GalactIc neIGHborHood 91 of tHe on tHe 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 accura- cies of a few micro arcseconds on hundreds of target stars or ~20 mas 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 approxi- mations 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).

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Panel rePorts—new worlds, new HorIzons 92 TABLE 2.1 General Capabilities: Theory, Astrometry, and Time-Domain Imaging and Spectroscopy Capability Specifications Scientific Applications Theory Support for theory must Simulations of IGM, CGM, galaxy formation accompany all major new Modeling galaxy feedback facilities Supermassive black hole spin and accretion Microphysics of accretion Connection to large-scale galaxy formation sπ ~ 1-10 mas Astrometry 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 Multiple visits, N ~ 100 Cepheid/RR Lyrae studies in Local Group imaging Optical transients, radio transient sky (almost totally unexplored) Time-domain R ~ 5,000 resolution Black hole masses spectroscopy NOTE: Acronyms are defined in Appendix C. TABLE 2.2 Capabilities: Short-Wavelength Bands (UV, X-Ray, Gamma-Ray) Capability Specifications Scientific Applications 10−13 cm−2 s−1 Gamma-ray F~ (broadband, 10% Dwarf galaxies and the dark matter particle spectroscopy resolution) 0.3-8 keV at R ≈ 300 X-ray imaging Mapping hot gas outflows, supernova remnants, FOV ~ 20′ and Dq ~ 5″ spectroscopy superbubbles Ilim ~ 10−14 ergs s−1 cm−2 arcmin−2 0.5-7 keV at 1-3 eV resolution Mapping hot plasma in SNRs, stellar winds, superbubbles Line detection sensitivity ~ 10× Chandra Black hole mass measurements 0.5-7 keV Dark matter indirect detection X-ray absorption 0.1-1 keV Abs-line spectroscopy of hot CGM R ≈ 3,000 spectroscopy Abs-line spectroscopy of hot ISM Aeff ~ 1,000 cm2 Dust grain mineralogy Inventory of abundant elements in ISM; obtaining kinematics of gas UV imaging 1,000-3,000 Å Mapping warm ISM and CGM R ≈ 1,000-2,000 Mapping galaxy outflows FOV ~ 20′ and Dq ~ 1″ Mapping hot plasma in the ISM and at the disk-halo Ilim ~ 100 LU (ph s−1 cm−2 sr−1) interface Now: STIS, GALEX Mapping star-forming regions Needed: Dq ~ 1″ 900-3,000 Å at R ≈ 30,000 UV spectroscopy Abs-line spectra of warm ISM, CGM Aeff ~ 104 cm2 Diagnose hot plasma in ISM and at disk-halo interface Now: STIS and COS (1,150-3,000 Å) Tomography of ISM, CGM Composition and kinematics of both hot and cold gas, Need: <1,030-3,000 Å excitation of H2 R = 30,000-100,000 to mlim ~ 19 NOTE: Acronyms are defined in Appendix C.

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rePort Panel GalactIc neIGHborHood 93 of tHe on tHe TABLE 2.3 Capabilities: Optical and Infrared Bands Capability Specifications Scientific Applications Optical B, V, R, I imaging polarimeter Mapping magnetic field in diffuse ISM polarimetry on large telescope Dp < 0.05% for mAB = 18 IR polarimetry J, H, K imaging polarimeter Mapping magnetic field in dense clouds Dp < 0.05% for mAB (K) = 18 0.01-0.1″ spatial resolution (HST successor) UV/optical/IR CMD-based star-formation histories for imaging V ~ 35 or 0.5 mag below main-sequence turnoff galactic neighborhood galaxies (Q3.1, Q4) Two-color photometry 5-10′ FOV Stable PSF and low backgrounds Substantial sky coverage (SDSS successor) V ~ 18-28 Multiband photometry Locating and identifying faint Milky Way halo Dq = 0.04″ substructure (Q3.2, Q4) including streams mlim ~ 25 and dwarf galaxies FOV ~ 10′ Dq = 0.04″ Probing Milky Way black hole mlim ~ 21 Proper motions of Milky Way stars Optical/near-IR R = 10,000-40,000, V ~ 18-19 Kinematics and abundances in Milky Way spectroscopy 500-1,000 multiobject spectrograph halo, dwarfs, and streams (Q3.2); power 105-106 stars 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 Kinematics of resolved populations in 10′ FOV, 100 multiobject spectrograph external galaxy halos (Q3.1) important to separate overlapping spatial components R = 400-2,000 Efficient preselection of EMP star candidates V ~ 18 (Q3.3) based on spectroscopically Large FOV (~all-sky coverage) measured bulk metallicity Obtain relative abundances to moderate precision R ≈ 3,500 NIR spectroscopy Black hole mass measurements Dq = 0.04″ 1-3 mm NOTE: Acronyms are defined in Appendix C.

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Panel rePorts—new worlds, new HorIzons 94 TABLE 2.4 Capabilities: Long Wavelengths (IR, Far-IR, Submillimeter, Radio) Capability Specifications Scientific Applications Far-IR imaging Now: Herschel (3.5-m warm telescope) Mapping dust distribution in nearby galaxies with ~5″ resolution and spectroscopy Study of major far-IR lines (63 mm [O I], 158 mm Needed: 5- to 10-m cold telescope [C II], 205 mm [N II] ) 2× better angular resolution >103× greater sensitivity Submillimeter Now: ALMA (bands 7-9) for high High-resolution line and continuum images of star- imaging and resolution forming regions’ high-excitation molecules, dust spectroscopy Needed: ~25-m telescope at high site Mapping star-forming clouds at galactic center Dq < 3″ at 350 mm Dust maps for nearby galaxies 350 mm-1 mm Potential for imaging black holes Polarimetry (linear) would be valuable Millimeter-wave Approved: ALMA High-resolution images of dust in star-forming imaging regions Needed: CARMA array receivers (1 mm) High-resolution images: molecular gas in star- forming regions Connecting galactic to extragalactic star formation Millimeter- Now: ALMA in S, CARMA in N Distribution of CO and other molecules in star- wave line and forming galaxies continuum Needed: CARMA array receivers (1 mm) Zeeman measures of B|| from CN mapping Needed: LMT large-format array receivers Molecular-cloud properties in different galactic environments Centimeter- Now: EVLA, GBT, Arecibo, ATA Improved H I maps, deep extragalactic imaging wave line and Needed: upgrade Arecibo and GBT to array Zeeman measurement of B|| using H I, OH continuum receivers Mapping synchrotron and free-free emission mapping Needed: upgrade ATA to 128 antennas Identifying and mapping new heavy molecules and transitions Gravity waves Detection Black hole masses NOTE: Acronyms are defined in Appendix C.