Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011) / Chapter Skim
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3 Report of the Panel on Galaxies Across Cosmic Time
3 Report of the Panel on Galaxies Across Cosmic Time
Pages 95-150
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From page 95... ...
-- St. Paul Irish Codex, translated by Robin Flower SUMMARY The study of galaxies across cosmic time encompasses the main constituents of the universe across 90 percent of its history, from the formation and evolution of structures such as galaxies, clusters of galaxies, and the "cosmic web" of intergalactic matter, to the stars, gas, dust, supermassive black holes, and dark matter of which they are composed.
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From page 96... ...
• How do black holes grow, radiate, and influence their surroundings? • What were the first objects to light up the universe and when did they do it?
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From page 97... ...
A Sloan Digital Sky Survey (SDSS) -size spectroscopic survey at z ~ 1-3 would provide essential information about the evolution of galaxy correlations and should provide essential clues to the process of galaxy formation and evolution.
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From page 98... ...
By quantifying the correlations of these properties with one another and with the larger-scale environment, astronomers can trace the evolution of galaxies and the baryons within them from the galaxies' origins to the present day. These detailed galaxy properties are accessible through rest-frame optical spectra that have suffi cient resolution to measure dynamical and stellar population parameters, sufficient continuum sensitivity to measure absorption lines, and sufficient emission-line sensitivity to measure low levels of star formation (see Figure 3.13 below in this report)
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From page 99... ...
, and higher-density probes provides the raw material for star formation. Neutral atomic gas in the circumgalactic medium likely feeds the growth of galaxy mass.
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How Do Black Holes Grow, Radiate, and Influence Their Surroundings? Supermassive black holes (SMBHs)
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From page 101... ...
The panel concluded: • It is very important to do complementary multiwavelength surveys to track the growth of black holes across cosmic time. A hard X-ray, all-sky survey for AGN is an essential complement to the deep pencil-beam surveys of active galaxies expected from the upcoming NuSTAR Explorer.
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From page 102... ...
The first objects to light up the universe could be stars, black holes, galaxies, and/or something less obvious, such as dark matter annihilation. What these ob jects are and when and where they formed are almost completely unknown.
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From page 103... ...
The panel concluded: • It is very important to do multiwavelength surveys to detect galaxies, quasars, and GRBs residing in the late stages of reionization at 6 < z < 8, including nearinfrared surveys for galaxies and quasars, hard X-ray or gamma-ray monitoring for GRBs, and time-variability surveys for supernovae or hypernovae.
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From page 104... ...
. Their critical constituents -- not only stars, gas, and dust, but also supermassive black holes and dark matter -- are strongly coupled to one another.
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From page 105... ...
millimeter VLBI, surveys Programs HST-like imaging of spectroscopic survey, Fermi, VERITAS, gravitational lenses, CO emission in HESS, MAGIC, HST, OIR MOS and IFU high-z star-forming IR and O III surveys, spectra of galaxies, galaxies, radio time-resolved UV black hole masses continuum survey, spectra with HST with ELT/ radio, long- far-IR spectroscopy COS, multiwavelength baseline radio imaging of luminous dusty surveys for AGN, of gravitational lenses, galaxies, JWST IFU multiwavelength deep multiband cluster spectra spectra blazars/AGN surveys XRSO,a high-resolution New Highly multiplexed XRSO, hard X-ray ELT, sensitive high-z facilities optical imaging of near-IR MOS on 8- survey telescope, 21-cm capability, GRB GWO,c ELT, X-ray needed lenses, OIR MOS and to 10-m, high-z H I observatory, hard X-ray IFUs on 8- to 10-m 21-cm, ELT, 4-m UV polarimeters and gamma-ray and telescopes, ELTb space telescope, far-IR optical time-domain spectra, large (sub) survey telescopes, CMB millimeter telescope, E-mode polarization, new XRSO, ELT IFU ALMA receivers Hydrodynamical simulations of Gpc3 volumes, studies of magnetohydrodynamics and plasma Theory needed physics, studies of gas flows and feedback, calculations incorporating radiative transfer, dynamical simulations within the hierarchical structure-formation context.
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From page 106... ...
. • The strong correlation observed between the stellar velocity dispersions in galaxies and the masses of their central black holes -- the so-called M-s rela tion -- indicates that the evolution of galaxies must be closely tied to the growth of supermassive black holes.
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From page 107... ...
Advances like these are driven by observations across a very broad wavelength range, by surveys and studies of individual objects, by temporal studies, and by theory. In the past decade, large surveys have provided unprecedented statistical coverage of galaxy properties, leading to such breakthroughs as the evidence for
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From page 108... ...
or for the study of nearby supermassive black holes (with the Panel on the Galactic Neighborhood) -- were resolved in discussions among the panel chairs, taking into account the continuity with the other material considered by each panel.
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From page 109... ...
The cosmic web of dark matter and gas condenses through gravitational instability, and galaxies form in the overdense regions that trace the underlying large-scale structure. SOURCE: Courtesy of Volker Springel, Heidelberg Institute for Theoretical Studies.
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From page 110... ...
The gas cloud in the center is distorted by the collision between the two clusters, whereas the dark matter has passed through the cluster gas without interacting. The clear separation between gas and dark matter is direct evidence that dark matter exists (as opposed to modified gravity)
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From page 111... ...
Active galaxies -- manifestations of accreting supermassive black holes -- were vastly more abundant and luminous in the past: their numbers and integrated luminosity peaked roughly when star formation was at its peak. Both black hole growth and star formation are implicated in feedback and in the evolution of structures on all scales.
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From page 112... ...
• How do black holes grow, radiate, and influence their surroundings? • What were the first objects to light up the universe and when did they do it?
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From page 113... ...
For example, the "fundamental plane" linking properties of elliptical galaxies or the Tully-Fisher relation for spirals, should be predicted by a successful theory of galaxy formation, as should the tight relationship between the mass of the central supermassive black hole and that of the host galaxy, as well as associations among stellar mass, total mass, and chemical composition of galaxies. Similar tight relationships are observed among the properties of clusters: for example, correlations among cluster mass and the luminosity and temperature of the hot intracluster gas.
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From page 114... ...
dark matter halos look quite similar, despite differing in mass by two orders of magnitude. The regions shown are at comparable contrast and resolution and correspond to approximately 2 virial diameters around each halo.
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From page 115... ...
Low-density regions are seen as dark bubbles surrounded by bright regions of hot, X-ray-emitting gas, believed to be generated by outbursts of activity from the central black hole. This may be direct evidence for feedback between black hole growth and cluster evolution.
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From page 116... ...
Here the panel outlines the highest-priority activities for the coming decade, on dark matter substructure, hot gas dynamics, galaxy properties, and black hole masses. Dark matter substructure and the overall shape of dark matter halos can be probed directly, on both galaxy and cluster scales, through its gravitational lens ing effects (Figure 3.8)
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From page 117... ...
A gravitational lensing experiment on the smaller scale of galaxies will constrain the dark matter substructure predicted by numerical simulations. SOURCE: Courtesy of NASA, N
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From page 118... ...
The panel concluded: • High-energy-resolution, high-throughput X-ray spectroscopic studies of groups and clusters to z ~ 2 are most important for understanding the dynamics, ionization and temperature structure, and metallicity of the hot intracluster gas, as
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From page 119... ...
It should be noted that, properly designed, the same large near-IR spectroscopic survey could serve the second key question as well. Measurements of supermassive black hole masses over the past decade have revealed a remarkable correlation between black hole mass and the stellar kinematics, mass, and luminosity of the galaxy bulge.
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From page 120... ...
Furthermore, mapping the spatially resolved emission from multiple rest-frame optical emission lines will trace out not only the distribution of star-forming regions but also the related gradients in chemical abundances. SOURCE: Reprinted by permission from Macmillan Publishers Ltd.: Nature, D.P.
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From page 121... ...
The panel therefore concluded that, in order to extend the M-s relation to low-mass local systems and massive high-redshift systems: • It is important to measure supermassive black hole masses in hundreds or even thousands of systems using spatially resolved spectroscopy with adaptive optics on ELTs, centimeter-wave maser observations with very long baseline arrays supplemented by large aperture dishes, and/or reverberation mapping of AGN. (See similar conclusion for the third key question, GCT 3.)
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From page 122... ...
Studying this epoch is crucial, since many properties of present-day galaxies were established at that time. While typical galaxies are expected to grow mainly by accreting material from the IGM, mergers are probably important for morphological transformations and central black hole growth and may drive the most powerful episodes of early star formation.
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From page 123... ...
I: Cold mode and hot cores, Monthly Notices of the Royal Astronomical Society 395:160-179, 2009, copyright 2009 Royal Astronomical Society.
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From page 124... ...
Over smaller areas one could probe substantially below L* , to study the pro genitors of typical galaxies today, with sufficient resolution to measure dynamical and stellar population parameters, sufficient continuum sensitivity to measure absorption lines, and sufficient emission-line sensitivity to measure low levels of star formation.
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From page 125... ...
observed in the Sloan Digital Sky Survey (SDSS) provides a well-sampled view of the local universe, but a much sparser sampling at large distances, during the key epoch of star formation (1 < z < 3, or look-back times of approximately 8 to 11 Gyr)
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From page 126... ...
Bernardi, M Blanton, et al., Stellar masses and star formation histories for 105 galaxies from the Sloan Digital Sky Survey, Monthly Notices of the Royal Astronomical Society: Letters 341(1)
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From page 127... ...
It should be noted that, properly designed, the same large near-IR spectroscopic survey could serve the first key question as well. • It is important to obtain spatially resolved spectroscopy of a representative sample of star-forming galaxies using IFUs on ground-based ELTs and NIRSpec on JWST.
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From page 128... ...
A complete inventory of cold gas in and around galaxies is also crucial for understanding baryon cycling. Molecular gas traced by CO, C I, and higher-density probes provides the raw material for star formation.
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From page 129... ...
Very Large Array map of CO (J = 2→1) emission from the z = 4.12 quasar PSS J2322+1944, tracing cold molecular gas in its host galaxy.
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From page 130... ...
For the most luminous and obscured sources, far-infrared spectroscopy could be used to break the degeneracy between the contributions of star formation and black hole accretion to the bolometric luminosity. The panel concluded: • It is very important to do sensitive radio and (sub)
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From page 131... ...
Progress to Date Supermassive black holes appear to play a unique -- and surprising -- role in shaping the structure and evolution of galaxies and clusters. If the basic idea of AGN feedback is borne out, it represents a truly remarkable example of multiscale physics -- an object a few light-minutes across influencing structure hundreds of thousands of light-years in extent.
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From page 132... ...
There has been remarkable progress in the development of knowledge of black hole demographics over the past decade. For AGN, this is principally due to deep panchromatic surveys from the radio to the X-ray.
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From page 133... ...
Very Long Baseline Array image of the jet being formed into a narrow beam within a hundred Schwarzschild radii of the black hole. The scale of each image is shown by white bars; 1,000 light-years is about 60 million times the distance from Earth to the Sun, and 0.1 light-year is about 1,000 Schwarzschild radii for M87's black hole mass of 3 × 108 M◉.
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From page 134... ...
. Major advances in the understanding of black holes and relativistic accretion disks will be enabled by a next-generation large X-ray telescope with high-throughput high-resolution spectrometers, sensi tive in the 0.5- to 10-keV band.
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From page 135... ...
. The supermassive black holes accrete from the surrounding gas, undergoing a quasar phase close to the merger event, and the associated feedback energy heats and expels a large fraction of the gas after the major merger.
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From page 136... ...
The results obtained with the recently approved Gravity and Extreme Magnetism Small Explorer are expected to motivate further investiga tions of X-ray polarization, and some other future X-ray missions have additional polarization capabilities. To understand the details of accretion onto supermassive black holes, jet for mation, and energy dissipation, the panel concluded: • It is most important to have sensitive X-ray spectroscopy of actively accreting black holes (AGN)
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From page 137... ...
Therefore, in order to seek evidence of black hole feedback, the panel concluded: • It is most important to measure turbulence and/or bulk flows using X-ray imaging spectroscopy of the ICM of nearby galaxy clusters and groups, with sufficient image quality, field of view, energy resolution, and signal to noise to provide ionization and velocity maps on the scale of the interaction between the AGN outflow (e.g., radio source)
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From page 138... ...
Optical and IR follow-up studies will also be useful to estimate black hole masses from broad-line widths. The panel concluded: • It is very important to do complementary multiwavelength surveys to track the growth of black holes across cosmic time.
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From page 139... ...
It will, however, be impossible to relate these SMBH mergers to the host galaxy properties without finding a significant number of electromagnetic counterparts. On balance, the panel concluded: • The search for gravitational radiation from merging supermassive black holes, at lower frequencies than are probed with the Laser Interferometer Gravity Observatory, is very important for an understanding of the buildup of supermassive black holes.
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From page 140... ...
Progress to Date The epoch between the last scattering of the cosmic microwave background radiation, at z ~ 1,100, and the current high-redshift frontier at z ~ 8, where the most distant quasars, GRBs, and galaxies have been observed, remains completely unexplored. This epoch contains the first stars, first galaxies, and first massive black holes.
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From page 141... ...
; Figure 3.20 summarizes current observational constraints on the inferred neutral hydrogen fraction of the IGM. Theoretical work suggests a general framework for the formation of the first objects: ΛCDM simulations indicate that the first objects formed as early as z ~ 40, out of gas that cooled via molecular hydrogen in the first dark matter halos.
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From page 142... ...
leave behind stellar mass seed black holes that accrete gas as mini-quasars. Depending on the efficiency with which ionizing photons escape into the IGM, the first halos might blow away their gas after a single episode of star formation and might not form any stars until
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From page 143... ...
Bottom: Simulation showing the X-rays produced by a black hole (white) , created when a first-generation star collapses, and their ionizing effect on nearby gas (blue)
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From page 144... ...
The ability to detect the first stars depends critically on the physics of star formation in a metal-free environment (e.g., initial mass function, star-formation efficiency, etc.) and the extent to which these first objects are clustered.
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From page 145... ...
is due to the Lyman break at rest-frame 912 Å. This is the highest redshift cosmic explosion detected to date.
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From page 146... ...
Absorption-line spectroscopy along sight lines toward the first objects -- be they stars, GRBs, or supernovae -- will allow the detection of the presence of met als and the ionization level throughout the epoch of reionization. Such observa tions necessarily require the collecting area and spectroscopic capability of an ELT, which can determine redshifts, stellar masses, and chemical compositions in early galaxies, quasars, and transient events.
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From page 147... ...
Theory is at the heart of astronomical inference, connecting observations to underlying physics within the context of a cohesive physical model. The past decade has seen great advances in theoretical aspects of galaxy formation and black hole astrophysics, particularly in the computational arena driven by technological advances.
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From page 148... ...
Studies of accretion disks, jets, and their interactions with ambient plasma require a better understanding of how magnetic fields channel and transport energy over a large dynamic range. Solving some of the key questions will require a better understanding of particle processes such as magnetic reconnection, particle acceleration, and cosmic-ray transport.
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From page 149... ...
All of these areas are informed by analytic theory, which is crucial for providing the basic physical underpinnings for more complex models. Together these aspects are fundamentally important for making progress toward understanding the universe, and the galaxies, clusters, black holes, gas, and dark matter within it.
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