9
Report of the Panel on Radio, Millimeter, and Submillimeter Astronomy from the Ground

SUMMARY

Astronomy at radio, millimeter, and submillimeter (RMS) wavelengths is poised for a decade of discoveries. The Atacama Large Millimeter Array (ALMA) will be commissioned in 2013, enabling detailed studies of galaxies, star formation, and planet-forming disks, with spectral coverage from 0.3 to 3 mm, at a resolution approaching 4 milli-arcseconds at the shortest wavelengths. Soon, the Expanded Very Large Array (EVLA) will have an order-of-magnitude more continuum sensitivity than the original Very Large Array (VLA), with continuous spectral coverage from 0.6 to 30 cm. The Herschel Space Observatory, with coverage from 60 to 670 μm, is delivering catalogs of tens of thousands of new “submillimeter-bright” galaxies. The Green Bank Telescope (GBT) operates over a broad range of centimeter and millimeter frequencies and has the potential for vastly improved mapping speeds with heterodyne and large-format bolometric-array cameras. With upgrades, the Very Long Baseline Array (VLBA) will improve astrometric distances critical to studies of star formation, galactic structure, and cosmology. It is possible that gravitational waves will be detected by timing arrays of pulsars, with the Arecibo Observatory playing a crucial role. The University Radio Observatories (UROs) will produce steady streams of excellent science, provide training grounds for graduate students, and remain at the cutting edge of science and technological development. The sizes of detector arrays at millimeter and submillimeter wavelengths and the computational capabilities of digital correlators are both experiencing exponential growth.

The foundation for further advances in this field must be laid in this decade.



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9 Report of the Panel on Radio, Millimeter, and Submillimeter Astronomy from the Ground SUMMARY Astronomy at radio, millimeter, and submillimeter (RMS) wavelengths is poised for a decade of discoveries. The Atacama Large Millimeter Array (ALMA) will be commissioned in 2013, enabling detailed studies of galaxies, star formation, and planet-forming disks, with spectral coverage from 0.3 to 3 mm, at a resolution ap- proaching 4 milli-arcseconds at the shortest wavelengths. Soon, the Expanded Very Large Array (EVLA) will have an order-of-magnitude more continuum sensitivity than the original Very Large Array (VLA), with continuous spectral coverage from 0.6 to 30 cm. The Herschel Space Observatory, with coverage from 60 to 670 mm, is delivering catalogs of tens of thousands of new “submillimeter-bright” galaxies. The Green Bank Telescope (GBT) operates over a broad range of centimeter and mil- limeter frequencies and has the potential for vastly improved mapping speeds with heterodyne and large-format bolometric-array cameras. With upgrades, the Very Long Baseline Array (VLBA) will improve astrometric distances critical to studies of star formation, galactic structure, and cosmology. It is possible that gravitational waves will be detected by timing arrays of pulsars, with the Arecibo Observatory playing a crucial role. The University Radio Observatories (UROs) will produce steady streams of excellent science, provide training grounds for graduate students, and remain at the cutting edge of science and technological development. The sizes of detector arrays at millimeter and submillimeter wavelengths and the computa- tional capabilities of digital correlators are both experiencing exponential growth. The foundation for further advances in this field must be laid in this decade. 439

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Panel rePorts—new worlds, new HorIzons 440 The crucial scientific questions and themes identified today can be addressed if the necessary steps are taken to lead to the instruments of tomorrow. RMS projects of modest cost will provide insights into the origins of the first sources of light that re-ionized the universe and led to the first galaxies. With truly large-format detector arrays on single-dish telescopes, large-scale surveys for galaxies forming stars intensely will inform the origin of the cosmic order observed today. An RMS project will provide insights into fundamental processes on the Sun and use the Sun as a laboratory for understanding the role of magnetic fields in astrophysical plasmas. Upgrades of modest cost to existing RMS facilities may allow the first discovery of gravitational waves and imaging of the event horizon around a black hole. The steps taken during this decade can lead to the next great advance in future decades, a telescope capable of studying the atomic gas flows that fed galaxies back in cosmic time and capable of studying the inner parts of circumstellar disks, where Earth-like planets may be forming. With continued, robust support for studies of the cosmic microwave background (CMB), RMS science extends from the Sun to recombination and the physics of inflation. The Panel on Radio, Millimeter, and Submillimeter Astronomy from the Ground has identified key capabilities that are needed to answer the science ques- tions posed by the five Astro2010 Science Frontiers Panels. By comparing those key capabilities to existing capabilities, the panel identified three new projects for mid-scale funding that will provide critical capabilities. The panel further identi- fied enhancements to existing or imminently available facilities that fulfill other requirements, and this report presents a balanced program with support for small facilities, technology development, laboratory astrophysics, theory, and algorithm development. Priorities and phasing are discussed in the panel report’s final section, “Recommendations.” Those recommendations are summarized here. Recommended New Facilities for Mid-Scale Funding The Hydrogen Epoch of Reionization Array (HERA) will provide unique in- sight into one of the last remaining unknown eras in the history of the universe. The panel recommends continued funding of the two pathfinders (collectively HERA-I) and a review mid-decade to decide whether to build HERA-II. The panel identified specific milestones to be met by HERA-I activities. If those are met, HERA-II is the panel’s top priority in this category of recommended new facilities for mid-scale funding. HERA-I requires about $5 million per year, as is currently being spent, and HERA-II construction is estimated to cost $85 million. The Frequency-Agile Solar Radiotelescope (FASR) will scan conditions in the chromosphere and corona across the full solar disk once a second, all day, every day. It is a vital complement to the Advanced Technology Solar Telescope (ATST) and provides essential ground truth for studies of magnetic fields on other stars. The

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 441 and froM tHe estimated construction cost for FASR is $100 million, and operations will cost $4 million per year, both of which the panel assumes will be evenly split between the National Science Foundation’s (NSF’s) Division of Astronomical Sciences (AST) in the Directorate for Mathematical and Physical Sciences and the Division of At- mospheric and Geophysical Sciences (AGS) in NSF’s Directorate for Geosciences. CCAT (formerly the Cornell-Caltech Atacama Telescope) will provide the capa- bility for rapid surveys of the submillimeter sky, essential for the optimal exploita- tion of ALMA. CCAT is a 25-m-diameter telescope located on a very high, dry site and equipped with megapixel detector arrays; it will address many of the questions posed by the Science Frontiers Panels. CCAT is estimated to cost $110 million, with $33 million coming from NSF. NSF’s share of operating expenses would be about $7.5 million per year, a net increase of $5 million per year, assuming that current funding for the Caltech Submillimeter Observatory (CSO) is recycled. FASR and CCAT have equal, and very high, priority in this category, but dif- ferent phasing. Development of Current and Imminent Activities Studies of the CMB have delivered much of the most valuable information about the universe at large. The panel strongly recommends a continued robust program at the current funding levels of ground-based CMB studies, with multiple approaches that are driven by individual investigators. An expansion of the Allen Telescope Array to 256 antennas (ATA-256) would significantly improve astronomers’ ability to find and study transient sources and to detect gravitational waves by timing an array of pulsars. The ATA can test ideas needed for the development of next-generation telescopes such as the Square Kilo- meter Array (SKA). The estimated cost of construction for the expansion is about $44 million. The panel recommends that NSF explore collaboration with other agencies and private foundations for the enhancement of ATA-42. The National Radio Astronomy Observatory (NRAO) telescopes (and soon, ALMA) provide a broad range of scientific capabilities needed to answer many of the SFP questions, but all will need instrument development, especially the comple- tion of frequency coverage, multibeam capability, and electronics improvements to enable much higher data rates. The panel recommends a sustained and substantial program to enhance the NRAO telescopes and ALMA capabilities, amounting to $90 million for NRAO and $30 million for the U.S. share for ALMA over the decade. The Arecibo telescope is essential for science with pulsars, which test general relativity, constrain the neutron star equation of state, and may lead to the detection of gravitational waves. The telescope can also make the deepest maps of galactic and extragalactic neutral hydrogen currently possible. A future multi-pixel up- grade would dramatically speed up surveys at centimeter wavelengths. The panel

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Panel rePorts—new worlds, new HorIzons 442 recommends that support for Arecibo be enhanced by $2 million per year over projected levels. The UROs provide cost-effective capabilities, testbeds for technology, and training grounds for young scientists. The panel recommends a modest enhance- ment ($2 million per year) in the budget for the current program, and it recom- mends that FASR ($2 million per year, starting in 2015) and CCAT (net $5 million per year, starting in about 2017) be operated under the URO program. Small Projects To achieve a balanced program, the panel recommends that a range of small and moderate projects be supported through a combination of funding from the Advanced Technologies and Instrumentation (ATI) program at NSF/AST and from NSF’s Major Research Instrumentation (MRI) program. Examples of such projects include an enhancement of the VLBI’s millimeter-wave capabilities to allow imag- ing of the event horizon around a black hole and multifeed receivers for the Com- bined Array for Research in Millimeter-wave Astronomy (CARMA). A program of technology development in a number of areas, and a focused program of laboratory astrophysics, are both vital needs. Support for theoretical work is crucial to realizing the investment in RMS facilities, as is a program of algorithm development. Both will allow observations to confront theory, essential to moving science forward. The panel recommends enhancements to NSF/AST’s ATI program of $1 million per year and an added program of laboratory astrophysics at $2 million per year. Looking to the Future The SKA has remarkable discovery potential, including studies of the epoch of reionization (SKA-low), determination of the gas content of galaxies at z of 1 to 2 (SKA-mid), and studies of the terrestrial-planet zones of planet-forming disks (SKA-high). However, substantial technology development is needed to define an affordable instrument. Many areas that the panel recommends for technology development will be crucial for this effort. The HERA project provides a develop- ment pathway for SKA-low, and the North American Array (NAA) project (part of NRAO development) develops technology for SKA-high. The panel recommends the continued development and exploration of options for realizing SKA-mid. THE SCIENCE CASE Here the panel identifies the RMS capabilities (in italic) that are needed to answer the science questions raised by the Science Frontiers Panels (SFPs) and summarizes them (as numbered below) in Table 9.1 at the end of this section and

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 443 and froM tHe also maps them in Figure 9.6 there. The panel lists only the SFP questions that RMS facilities can address. It also poses some additional, unnumbered, relevant questions. In later sections, the panel matches these capabilities to specific activities and concludes with recommendations. Cosmology and Fundamental Physics (CFP) Q1: How did the universe begin? The CFP report requires a suite of instruments that characterize the cosmic microwave background (CMB) to determine proper- ties of the early universe. CMB observations probe the potential-energy function of the primordial field (or fields), the type of primordial fluctuation (adiabatic or iso-curvature), whether the fluctuations were Gaussian, and the degree to which gravitational waves (tensors) played a role in the infant universe. CMB observations may yield the only observational constraint on theories of quantum gravity. These measurements will complement the Planck satellite, which will have exquisite sen- sitivity to temperature fluctuations for 2 < l < 2,500, where l is the multi-pole index of a spherical harmonic; the corresponding angular scale is q ~ 180/l in degrees. With new CMB polarization measurements in the range 2 < l < 200, one can find or limit primordial gravitational waves and probe reionization. The polarization for 200 < l < 3,500 sheds light on early-universe physics, neutrino mass, and the helium abundance. The temperature spectrum for 1,000 < l < 10,000 should be pursued from the ground to understand the high-l tail of the primary CMB spec- trum and, for example, to identify clusters through the Sunyaev-Zel’dovich effect (SZE). The search for large-angular-scale B-modes (from tensor fluctuations) will discover them or decrease the current limit on the tensor-to-scalar ratio from about 0.3 to 0.01. These techniques lay the foundation for a future satellite focusing on polarization. Q2: Why is the universe accelerating? It is known that the universe is accelerating but not why or how. To make progress, the z-dependence of the dark-energy equation of state, w(z), and the Hubble parameter, h(z), must be better determined. Inti- mately related to these are possible changes in the gravitational coupling constant (G) and the growth rate of structure. The CFP report calls for precision tests of general relativity, which is central to our understanding of cosmology. Studies of pulsars in relativistic binary systems provide the strongest constraints on possible changes in G, and timing may allow detection of gravitational radiation from the early universe. Our capabilities to find relativistic binary pulsars and time them must be enhanced. Additional tests will come with measurements of the supermas- sive black hole (SMBH) in the center of our galaxy with ultrahigh spatial resolution and through gravitational lensing of radio by SMBHs in distant galaxies, requiring sensitive centimeter-wave imaging. The growth of structure will be measured a

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Panel rePorts—new worlds, new HorIzons 444 number of ways with the CMB. Through the SZE, measurements of clusters can provide a mass-selected, almost z-independent sample of clusters of galaxies over large regions of sky, tracing the development of clusters from the early to nearby universe. Q4: What are the properties of neutrinos? The presence of relativistic neutrinos affects the growth of structure at early times. High-l CMB-lensing B-modes are particularly sensitive to this effect, as are other aspects of the measurements. CMB experiments are expected to determine the sum of neutrino masses to 0.05 eV. A vigorous ground-based CMB program is a requirement in this area. Discovery area: Gravitational wave astronomy—listening to the universe. The CFP report calls for pulsar timing arrays to probe the nanohertz frequency range to detect the stochastic background of gravitational waves from SMBH binaries (Figure 9.1). A project focusing on this goal is discussed in the PAG report, and so only the consequences for RMS facilities are summarized here. A rough require- ment for gravitational-wave detection is careful timing of the arrival of pulses from a set of ~20 stable, millisecond pulsars distributed over the sky to an accuracy of 100 ns over a period of 5 years. This task may be divided into two parts: discov- ery and timing. Discovery requires large collecting area, wide bandwidths, high- throughput back-end electronics, and computational power. In the United States, pulsar-discovery capabilities exist primarily at Arecibo and the GBT. They must be sustained and upgraded. Timing requires about a day per week of observations by a telescope with 104 m2 collecting area and with back-end hardware and software capable of coherent dispersion removal. Observations across a wide bandwidth or at multiple simultaneous frequencies are essential to correct for effects due to interstellar dispersion. Timing requires either the reallocation of existing facilities or ideally a facility with a large fraction of the time dedicated to timing. Galaxies Across Cosmic Time (GCT) Enabled in part by RMS observations with new facilities and instrument tech- nologies, an understanding of the formation and evolution of galaxies and large- scale structure is beginning to form. However, fundamental questions remain. The observed structures of galactic dark-matter halos challenge structure-formation theories. The interaction of gas and stars in the galaxy-building process is poorly understood, driving a need for inventories of the cold atomic and molecular gas contents of galaxies. Supermassive black-hole growth and feedback must be charac- terized to understand the correlation between black-hole masses and stellar-bulge velocity dispersions. At the highest redshifts, the nature of the first objects that reionized the universe remains unconstrained.

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 445 and froM tHe FIGURE 9.1 Black holes orbiting one another produce ripples in space-time or gravitational waves. Left: The ripples from a single supermassive black-hole binary cause the signals traveling from two pulsars to the tele- scope to arrive faster or slower than expected if they were traveling through flat space. Right: The superposition of many supermassive black-hole binaries distributed throughout the universe gives rise to a low-frequency stochastic background. This background will induce correlated shifts in the arrival times of an array of pulsars. SOURCE: Left: D. Backer/JPL/NASA. Right: David Champion, Max-Planck-Institut für Radioastronomie. Q1: How do cosmic structures form and evolve? RMS facilities can characterize the shapes and substructures of galactic halos using high-angular-resolution observa- tions of gravitational lensing. The EVLA will find hundreds of lenses, but ultimately approximately 106 lenses will be needed, requiring a capability for very sensitive centimeter-wave imaging. Unbiased by redshift, SZE surveys associated with the CMB program will discover large samples of clusters out to their epoch of forma- tion. Understanding the physics of these clusters will require multiwavelength ob- servations, including detailed centimeter and millimeter observations (Figure 9.2). Q2: How do baryons cycle into and out of galaxies, and what do they do while they are there? A comprehensive model of galaxy formation requires an understand- ing of accretion, mergers, and evolution of the gas in galaxies from the formation of the first galaxies to the present day. For studies of dusty, distant, star-forming galaxies, large-area (tens of square degrees), continuum submillimeter surveys will sample the galaxy luminosity function over a broad range of redshifts. Large, single-dish submillimeter and millimeter telescopes equipped with large-format detector arrays are needed. To characterize the gas associated with star formation, spectroscopy of CI, CO, and molecular tracers of dense gas is essential. The current ALMA and EVLA facilities will offer broad but incomplete coverage in redshift for the detection of CO in distant galaxies. New facilities and upgrades can enable fast

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Panel rePorts—new worlds, new HorIzons 446 FIGURE 9.2 Deep submillimeter extragalactic image in the GOODS-N region obtained with the Hershel Space Observatory SPIRE camera. The three images at the left are in three bands, which are coded in those colors in the large composite. All the sources are dusty galaxies. Bluer galaxies are warmer and/or more nearby, while redder galaxies are cooler and/or at higher redshift. The submillimeter sky is extremely rich in galaxies—thousands of (unresolved) high-redshift galaxies appear, and the image is highly confusion-limited. SOURCE: Courtesy of the European Space Agency and the SPIRE and HerMES consortia. spectroscopic follow-up observations of galaxies detected in continuum surveys, particularly in the z = 2 to 4 range. This goal requires complete submillimeter- millimeter frequency coverage, broadband correlators, and multiobject spectro- scopic capability for single-dish telescopes. Large-scale intergalactic gas flows and early accretion onto galaxies will consist largely of atomic hydrogen. H I 21-cm line observations can assess the total masses and atomic gas contents of galax- ies, circumgalactic streams, and the cosmic web, mapping the gaseous origins of galaxies. The current suite of centimeter-wave telescopes are limited to observing H I structures in the local (z < 0.2) universe. Detecting H I in galaxies at redshifts as high as z ~ 1 will require very sensitive centimeter-wave imaging, requiring a

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 447 and froM tHe large increase in collecting area over current facilities and substantially increased correlator capabilities. Q3: How do black holes grow, radiate, and influence their surroundings? Supermassive black holes are ubiquitous in the centers of galaxies. However, much still needs to be learned about how they become active galactic nuclei (AGN), and how they interact with their host galaxies. Ultrahigh-spatial-resolution interferometric observations can resolve regions close to AGN: jet-formation mechanisms can be constrained by VLBA and submillimeter VLBI observations of blazars, and submillimeter VLBI will enable studies of circumnuclear disks in galaxies, including our own. Q4: What were the first objects to light up the universe, and when did they do it? The epoch of reionization (EoR) is the frontier in understanding galaxy formation. Understanding cosmic reionization at z > 6 requires mapping the H I distribu- tion via the hyperfine transition, which is redshifted to meter-wavelengths for z > 5. The first-generation instruments (PAPER, MWA, EDGES, and international efforts) are designed to detect fluctuations and constrain the redshift range of reionization. Precise measurement of the power spectrum of H I emission re- quires a sensitive meter-wave array with a factor-of-10 larger collecting area (Aeff ~ 105 m2). Imaging hydrogen structures will require another order of magnitude in collecting area (Aeff ~ 106 m2). Whether stars in galaxies or other sources are primarily responsible for cosmic reionization remains controversial. The roughly constant apparent brightness of dusty galaxies of a given luminosity as a function of redshift in submillimeter and millimeter bands provides an advantage for fast surveys at millimeter/submillimeter wavelengths. Observations of redshifted CO, C II, N II, and O I have the potential to measure the radiative cooling of galaxies in the later stages of the EoR, requiring complete wavelength coverage in spectral windows accessible from the ground. The Galactic Neighborhood (GAN) The galactic neighborhood (GAN) science frontier covers galaxy build-up and evolution back to z ~ 0.1. RMS facilities are well suited for GAN studies through their ability to probe cold flows, star formation, the environment of SgrA*, and magnetic fields. RMS interferometers provide high resolution and high astrometric accuracy (Figure 9.3). Q1: What are the flows of matter and energy in the circumgalactic medium? Nearly all massive galaxies are accreting material, observed as H I and stellar streamers from tidally disrupted satellite galaxies. Radio facilities are required to assess the kinematics and distributions of H I on both large (low-resolution) and small (high-

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Panel rePorts—new worlds, new HorIzons 448 FIGURE 9.3 Neutral hydrogen (H I) gas in the Virgo cluster of galaxies taken with the VLA, colored by velocity (galaxy sizes have been increased by a factor of 10 to make them more visible). The background image is from ROSAT. SOURCE: NRAO/AUI and Chung et al., Columbia University. resolution) scales. Feedback in galaxies from star formation or AGN activity may enhance or halt additional star formation. By imaging radio synchrotron emission, H I, and molecular lines, one can assess the impact of feedback on the surrounding medium. Sensitive, low-resolution observations with radio facilities play a crucial role in studying feedback on large scales, but efficient, high-resolution, centimeter- wave capabilities are needed to provide maps of energy deposition into the ISM.

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 449 and froM tHe Q2: What controls the mass-energy-chemical cycles within galaxies? RMS facilities play a critical role in studying the build-up of stellar mass by gas inflow and ac- cretion over cosmic time. H I is a prime tracer of the inflow of gas into galaxies, some of which becomes star-forming H2 clouds. Observations of dust emission and molecular lines (millimeter-submillimeter) trace the gas, and synchrotron emission from supernovae traces star-formation rates. Very sensitive 2-cm imaging and fast surveys at submillimeter wavelengths and full wavelength coverage from meter to submillimeter will be required to achieve the goals. Q4: What are the connections between dark and luminous matter? The H I disks of galaxies extend well beyond the stellar light of galaxies and were instrumental in providing evidence of dark matter. Sensitive, high-resolution H I observations are required to trace the kinematics of the faint, dark-matter-rich dwarf galaxies and the outer parts of disk galaxies. Accurate measurements of the properties of SMBHs and their interplay with their immediate gaseous environments are re- quired to understand how SMBHs fit into the build-up and evolution of galaxies, including nuclear star-forming rings. These science goals require high-resolution (<0.05 ″) radio and millimeter observations to probe the non-thermal emission and state of the star-forming gas, respectively. Ultrahigh resolution at millimeter/ submillimeter wavelengths tantalizes us with the possibility of imaging the event horizon of Sgr A*. Discovery area: time-domain astronomy. Temporal RMS observations are a largely unexplored area of astronomy likely to show significant progress in the next decade. Such observations require a sensitive, dedicated instrument designed to scan the available sky rapidly enough to sample variable (or moving) objects. A dedicated transient-search telescope covering a wide range of wavelengths is needed to ex- plore this discovery space. Discovery area: astrometry. One of the great strengths of RMS interferometers is the direct detection of electromagnetic phase and the ability to do astrometry routinely to a fraction of a beamwidth. Parallaxes and proper motions can be measured within the galaxy, and for galaxies within the Local Group and beyond, allowing for a better assessment of dynamics and everything that comes with better distances. An ultrahigh-resolution capability with improved sensitivity is needed for precision astrometry. Stars and Stellar Evolution (SSE) Q1: How do rotation and magnetic fields affect stars? For many years, radio obser- vations have provided clues to magnetically driven solar and stellar activity. In

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Panel rePorts—new worlds, new HorIzons 490 FIGURE 9.17 Submillimeter Array 1.3-mm spectra from three dust cores (each plotted in a different color offset in the y-axis for clarity) in the massive star-forming region NGC6334I (sources are separated by only 2 arcseconds or about 4,000 AU) show tremendous chemical complexity over small size scales as well as a large fraction of unidentified lines. SOURCE: Todd R. Hunter, National Radio Astronomy Observatory. provided some directed funding for laboratory astrophysics in support of a few missions, for example the Herschel Space Observatory, but that funding has been limited and of relatively short duration. In the coming decade, it is critically im- portant that the astronomical community find a way to provide sustained support for laboratory astrophysics, potentially through cross-divisional NSF funding ini- tiatives or directed funding lines within the astronomy division. The full scientific potential of the next generation of RMS facilities will be severely compromised without a commensurate dedication of resources to laboratory astrophysics. The panel recommends a program funded at $2 million per year.

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 491 and froM tHe Theory for RMS-Related Science The panel lists below in two categories some theoretical needs that are espe- cially important for RMS science. The theoretical developments required for extracting science from RMS obser- vations are the following: • Theoretical calculations of fluctuations in the intensity and polarization of the cosmic microwave background are an inherent part of deriving cosmological constraints from the CMB data. • Numerical simulations of cosmic reionization are needed to predict ob- servables for redshifted 21-cm experiments (especially for the first stage, HERA-I). Given the potential complexity of foreground removal and accurate calibration, without such modeling the observational measurements of statistical properties of the cosmological signal (such as the power spectrum) will remain inconclusive until actual imaging capabilities are developed. • Full three-dimensional modeling of circumgalactic environments is crucial for the correct interpretation of the spectroscopic data. Without such modeling, differentiation between hot-gas accretion, outflows, virial shocks, and gas flows along filaments is extremely difficult or impossible. • Chemo-dynamical models of star-forming regions that trace the simulta- neous evolution of density, gas and dust temperature, and molecular abundances through the evolution of a particular dynamical model are necessary for a correct interpretation of molecular-line observations. • Spectropolarimetric and magnetohydrodynamic forward modeling of the solar atmosphere in three dimensions will be essential to exploit the full potential of FASR data. • Interference mitigation will be critical, especially for low-frequency observations. In several other areas, major theoretical development will be required to fully realize the investment in an RMS facility: • Modeling cosmological structure formation in representative cosmological volumes with resolution adequate to include the most of the important physical processes in the ISM. • Modeling black hole accretion on a wide range of scales, from the inner edges of accretion disks to global galactic environments. • Theoretical studies and numerical modeling of MHD processes on a wide range of scales, from the small-scale physics of reconnection and particle accelera- tion to the effects of magnetic fields on galactic and extragalactic scales.

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Panel rePorts—new worlds, new HorIzons 492 • Numerical modeling of radiative transfer in galactic simulations, in simula- tions of star-forming regions, and in models of black-hole environments. • Precision modeling of stellar pulsations. • Theoretical studies and numerical modeling of planet formation, evolution, and dynamics in a range of environments. • Modeling of radiative and dynamical processes in planetary atmospheres. • Modeling of gravitational-wave signatures expected from stochastic back- grounds and from individual sources which could be detected by pulsar-timing arrays. Algorithm Development Many of the scientific goals for the next decade rely on the development of new algorithms that will facilitate the processing of large datasets, allow the discovery of weak signals, and foster cross-disciplinary data sharing. The most critical needs are as follows: • Foreground removal for epoch of reionization studies. • Optimal detection and characterization of gravitational-wave signals in pulsar data. • Computationally efficient pulsar-search algorithms to handle large amounts of data with sensitivity to the most relativistic binary systems and weak transients. Real-time search algorithms are imperative given the expected data rates of new correlators and multi-pixel receivers. • Spectral-line-analysis tools that aid in the identification of lines, extraction of line parameters, and analyses of physical conditions using laboratory astrophys- ics results and radiative-transfer algorithms. • Automated source-detection algorithms in the spectral domain to aid in the creation of source catalogs in formats compatible with Virtual Observatory standards. • Imaging algorithms that keep pace with the cutting edge of possible data rates through parallelization and high-performance computing possibilities. A crucial need for RMS is greater access to high-speed data transmission for data acquisition and retrieval as well as access to long-term storage. Many of the proposed facilities can produce data at rates approaching a petabyte an hour. Innovative solutions for storage and public access, in keeping with observatory policies, are necessary. Strong partnerships between the NSF-AST division and the NSF Office of Cyberinfrastructure, as well as international agreements, will facilitate these goals.

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 493 and froM tHe Spectrum Management The radio spectrum is a precious resource for radio astronomy and for com- munication in the modern world. Without continued vigilance to protect some of this resource for passive scientific use, radio astronomy from Earth’s surface will become increasingly difficult. The traditional approach of seeking protection for small, defined segments of the spectrum for radio astronomy is no longer adequate because of the wide bandwidths needed for sensitivity and the broad frequency coverage needed for spectral-line studies at high redshifts. Resources must be made available to develop modern technologies for radio-interference mitigation and for sharing the spectrum through time- and frequency-multiplexing methods. Looking to the Future The Square Kilometer Array is seen as the next-generation centimeter-wave and meter-wave telescope, which would address many fundamental science ques- tions. This project has already garnered significant international support, with 55 institutions in 19 countries participating. The 2001 decadal survey recommended that the U.S. participate in a program of technology development funded at $22 million; $12 million has been funded, starting in 2007. Over the past decade, the SKA has evolved into three instruments covering three wavelength regimes: SKA-low (1 to 3 m); SKA-mid (3 to 100 cm); and SKA- high (0.6 to 3.0 cm). The panel believes that it is very important for the United States to play a role in this international project. However, based on the informa- tion received from the projects and from independent analysis, none of the parts of this project have reached maturity sufficient to recommend construction at this time. Defining the way forward in this context requires a mix of technology development, demonstrator projects (e.g., LWA, MWA, PAPER, LOFAR, MeerKAT, ASKAP), and careful consideration of priorities. The results of the demonstrators will not be available for a number of years. The long-wavelength (1.0 to 3 m) part of the spectrum covered by SKA-low provides the only way to study the process of reionization (H I at z = 5 to 14); through that, it is one of the most promising ways to study the first luminous objects (GCT 4). The HERA activity provides a step-by-step path for the U.S. com- munity to lead in this exciting aspect of SKA science. It lays out a sequence of inter- mediate science and associated technology goals that address this key science area. The mid-wavelength (3 to 100 cm) part of SKA provides the capability for very sensitive centimeter-wave imaging. SKA-mid is essential to study the role of atomic gas in galaxy evolution (GCT 2; GAN 1); it could provide spectroscopic imaging of the H I emission for a billion galaxies out to z ~ 1. This cannot be done with present facilities and is strong justification in itself for this ambitious instrument. SKA-mid

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Panel rePorts—new worlds, new HorIzons 494 could address other key scientific questions as well: it would enable a powerful pulsar machine that would enable a census of the galactic pulsar population (SSE 4), test general relativity in the strong-field regime (CFP 2), and almost certainly detect low-frequency gravitational waves and constrain gravitational-wave sources (CFP 1, Discovery). It would provide excellent sensitivity to the transient radio sky (SSE Discovery). In spite of the compelling science case for it, the panel finds that there are sub- stantial issues of technical readiness and cost for SKA-mid. The total construction cost of the project, already $2.2 billion in the project’s estimate, was raised to $5.9 billion by independent analysis. SKA-mid was considered not technologically ready in the independent analysis, and the panel concurs. Further development and study of alternative options for this wavelength range are needed and could be funded in open competition within the ATI program, potentially in conjunction with other international efforts. Pathfinders, such as ATA-256, could help to test technical concepts that would lead to the final design of the SKA. Alternative approaches to constraining dark energy, such as 21-cm intensity mapping, could be explored to see if they can be useful on shorter timescales and at lower cost. The panel recom- mends revisiting the SKA design costs in 5 years to assess end-of-decade feasibility. The short-wavelength (0.6 to 3.0 cm) part of SKA helps constrain dark energy (CFP 2), dark matter (CFP 3), galaxy evolution via CO and other molecules at z above 1.3 (GCT 2), planet formation (PSF 2), and the ends of massive stars (SSE 3). Because of the U.S. heritage with EVLA, GBT, VLBA, and ALMA, it is natural for the United States to build on these in developing this part of SKA. A modest program of technology development and prototyping should begin in this decade. The NAA activity discussed above provides an attractive way to proceed. RECOMMENDATIONS The panel recommends a program with three new major initiatives for mid- scale funding, upgrades to existing and imminent facilities, and increased funding for smaller facilities. The panel identifies a need for technology development in four main areas and an interdisciplinary laboratory astrophysics program, along with theory and algorithm development relevant to RMS science. The panel’s recom- mendations are made in the context of the following assumptions: a 7 percent per year increase in the NSF-AST budget and the augmentation of a funding line for mid-scale construction projects of at least $20 million per year. The panel recom- mends no projects for MREFC funding.

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 495 and froM tHe Major New Initiatives The major new initiatives are HERA, CCAT, and FASR. In terms of scientific importance, the panel ranks HERA first, with CCAT and FASR tied and close behind. However, there are issues of readiness that require a distinction between ranking and phasing of project starts. Of these, FASR is the most ready to proceed and, as the panel has noted, has excellent prospects for cross-directorate funding. The panel recommends that a funding strategy for FASR be developed with core contributions from both NSF- AST and NSF-AGS (the panel assumes here an even split). The FASR Pathfinder proposed by the project to AGS ($8 million) would be a good way to begin and resolve any remaining cost questions, but it should proceed in a manner that is ultimately compatible with the full implementation of FASR. The $50 million from AST would come from the mid-scale line starting in 2010 and ending in 2015. The 50 percent AST share of operations ($2 million per year) could come from an increase in the URO budget. CCAT is also far along in design. The project, a consortium of U.S. and foreign institutions, estimated a total cost of $110 million, and the independent estimate was $138 million for construction. Of this, the consortium requested only $33 million from NSF. The panel recommends proceeding with this project as soon as the design is finalized and the consortium has the balance of the funding or suit- able guarantees. There is substantial urgency in this project, because it will provide sources to optimize use of ALMA. CCAT would be an ideal candidate for funding by a mid-scale funding source when it becomes available, but it should start by 2012. It should phase in after FASR and complete funding by 2017. Operations costs from NSF will be $7.5 million per year, but shutting the CSO will save $2.5 million per year, and so $5 million per year extra will be needed in the URO budget by 2018 (see below). HERA has the panel’s top science ranking, but it comes in three phases. The first phase, HERA-I, is underway with two parallel efforts engaged in testing tech- niques. The panel strongly recommends continued funding for both these efforts at a combined rate of approximately $5 million per year to about 2015, at which time a review would be needed. If the HERA-I projects achieve certain milestones, the panel would strongly favor funding of HERA-II, or a similar project selected in open competition, from the mid-scale funding line. The milestones are demonstra- tion of successful techniques for calibration and foreground removal; detection of the power spectrum of H I in the epoch of reionization; a decision on the optimum design for HERA-II; and development of a full proposal for HERA-II with cred- ible costs. The current estimates of cost for a mid-decade start for HERA-II range from $80 million to $115 million, but they depend strongly on future correlator developments. There should be further technology development toward HERA-III,

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Panel rePorts—new worlds, new HorIzons 496 which would be a future-decade project. For HERA-I, the costs are a continuation of current funding, and so the panel counts only HERA-II as new funding and assumes a total cost of $85 million from the mid-scale line, starting in 2015 and ending in 2020. The panel assumes no operations funding this decade. Sustaining and Upgrading Current and Imminent Facilities The panel’s top priority in the facilities area is continued funding of a vigorous, diverse program of ground-based research on the cosmic microwave background. Detection of the so-called B-modes, which trace primordial gravitational waves, is a primary goal. The program should also constrain cosmological parameters, determine or limit the sum of neutrino masses, and measure large-scale structure. Since this is an ongoing program, it does not represent new costs, but the panel emphasizes its absolute importance. The current ATA-42 is in serious need of funding. This project has pioneered the concept of large arrays of inexpensive antennas with broadband imaging re- sponse. Its current NSF support is inadequate to keep the current array running, much less to continue the technological evolution of the array to 256 antennas. The observing capabilities provided by the ATA-256 would provide major advances in the ability to find transients and detect gravitational waves. Moreover, this project can provide valuable technological developments for a mid-range SKA, in the area of wideband feeds, large-field imaging, and large-correlator development. The panel could not fit the funding for ATA-256 into a $20 million per year mid-scale line, but it would be the preferred back-up for such funding should HERA-I not meet its milestones. ATA-256 may be able to attract further funding by private foundations or other agencies. The panel recommends an effort to explore ways to move forward with a modest investment of NSF funds. This is the second priority in this category. The panel recommends a regular program of upgrades for existing facilities, including ALMA, NRAO facilities, Arecibo, and elements of the UROs program. These upgrades will provide some of the capabilities identified as needed to an- swer the science questions. In particular, such upgrades are the most cost-effective way to obtain the capability for efficient high-resolution imaging at centimeter wavelengths and improved sensitivity with ultrahigh resolution (see Table 9.1). Multifeed arrays on the GBT and CARMA provide test beds for new techniques. Convincing cases were made for a total of $90 million over the decade for each of ALMA (the U.S. share is $30 million) and NRAO. The current UROs need more operating funds to improve their utility to the larger community (the panel recom- mends an extra $2 million per year) and further increases (ending the decade with an increment of $9 million per year) once FASR and CCAT become operational.

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 497 and froM tHe Because of the importance of Arecibo for pulsar timing, the panel recommends restoring $2 million per year in funding to its baseline budget. Small Projects Keeping a balance between large projects and national/international facilities and smaller projects is vitally important. Examples of excellent projects of this kind are the enhancement of millimeter-wave VLBI to create the Event Horizon Telescope, adding the huge collecting area of ALMA, and the addition of multifeed receivers to the CARMA telescopes. The panel recommends a total of $25 million for this effort over the decade, most likely funded by ATI or MRI. The RMS System Funding for user support and archive exploitation is important to the opera- tion of the RMS system. The panel also recommends enhancements to the ATI program (by $1 million per year) that will allow more technology development for the future. The panel also recommends a program of laboratory astrophysics ($2 million per year) in which similar programs can be evaluated in context. RMS- related science depends heavily on laboratory and theoretical advances. Strategic theory and algorithm development should be supported to maximize the return on investments in facilities. Summary With a combination of new facilities and the sustenance and invigoration of existing programs and facilities, almost all the RMS capabilities needed to answer the science questions posed by the Astro2010 Science Frontiers Panels can be real- ized (Table 9.4, Figure 9.18). The most notable exception is the capability for very sensitive centimeter-wave imaging, needed for the study of H I at redshifts of 1 to 2. That requires something like SKA-mid, and the panel recommends some steps toward that goal. The panel summarizes in Table 9.5 the additional costs to NSF- AST for construction and operations. Table 9.5 indicates which items would be suitable for mid-scale funding and prioritizes projects costing at least $30 million.

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Panel rePorts—new worlds, new HorIzons 498 TABLE 9.4 Needed RMS Capabilities and the Panel’s Recommended Actions Capability Recommended Action Cosmic microwave background program Continue successful program Sensitive meter-wave array Continue HERA-I, fund HERA-II mid-decade Solar radio telescope Construct FASR Fast millimeter/submillimeter surveys Participate in construction, operations of CCAT Fast centimeter surveys Enhance ATA, GBT, Arecibo Efficient high-resolution imaging at centimeter/millimeter Enhance EVLA, ALMA, CARMA Very sensitive centimeter imaging Cannot meet this decade, technology development Dedicated pulsar timing, transients Enhance ATA, Arecibo, GBT Ultrahigh resolution Enhance VLBA, millimeter-wave VLBI Complete wavelength coverage Enhance ALMA FIGURE 9.18 Mapping of re- quired capabilities to new ini- tiatives, upgrades of existing facilities, and continuation of successful programs. Dashed arrow indicates that need can- not be met this decade. 9-18.eps bitmap

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radIo, MIllIMeter, subMIllIMeter astronoMy Ground 499 and froM tHe TABLE 9.5 Added Costs to NSF-AST for Construction and Operations (FY2009 million dollars) Construction Action (total) Priority Operations (per year) 85a Continue HERA-I, construct HERA-II 1 0 50a Construct FASR (50 percent AST) 2, tie 2, starting 2015 33a Participate in construction, operations of CCAT 2, tie 5, starting 2017 44b Enhance ATA if possible 4 3, increasing to 6 in 2015 90c Enhance GBT, EVLA, VLBA 5, tie 1 30c Enhance ALMA 5, tie 25c Enhance CARMA, EHT Enhance UROs support 2 Enhance Arecibo support, if possible 2, starting in 2012 Enhance ATI 1 Laboratory astrophysics 2 Total over decade 357 131 aMid-scalefunding. bMid-scaleor other funds for upgrades. cSome elements could be mid-scale instruments; others could be MRI or ATI.

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