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Appendix D: Report of the Panel on Galaxies
Pages 264-283

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From page 264...
... The dark matter structures are permeated by a tenuous circumgalactic, intracluster, and intergalactic medium, made of primordial hydrogen and helium that may eventually join galaxies and form into stars, as well as of enriched gas that carries the products of previous stellar generations back into intergalactic space. The goal of the field of galaxy formation and evolution is to achieve a predictive formulation of the assembly histories of galaxies and their dark matter halos, together with the evolution of their stellar populations, black holes, physical structures, chemical content, and circumgalactic, intracluster and intergalactic media.
From page 265...
... Question D-Q4. How do the histories of galaxies and their dark matter halos shape their observable properties?
From page 266...
... While the stellar mass of central galaxies is tightly correlated with the halo mass, star formation in satellite galaxies appears to become quenched after the stars enter the parent galaxy's halo, which suppresses their stellar mass growth and reddens their colors relative to isolated systems. The observed correlation between SMBH mass and host galaxy stellar mass, well established for galaxies with Mstars > 1011 M⊙, hints at coupled evolution, with BH growth regulating stellar mass or vice versa.
From page 267...
... The ultra-faint dwarf satellites of the Milky Way and M31 probe the low-mass threshold of galaxy formation, with stellar populations that may have formed before reionization. Dynamical measurements of these galaxies show many orders of magnitude range in stellar mass over a narrow range of halo mass, Mhalo ~ 108–1010 M⊙, suggesting that the final stellar masses are sensitive to intersections between star formation histories, feedback processes, dark matter assembly, and, possibly, dark matter physics, in ways that are still not fully understood.
From page 268...
... the number of low-mass galaxies in the early PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION D-5
From page 271...
... Cosmological simulations require feedback from stellar winds/supernovae and from SMBHs at the low and high galaxy mass ends, respectively, to regulate the baryonic accretion onto dark matter halos and reduce star formation efficiency. Models predict that, depending on the halo mass, a catastrophic loss of cool ISM from the galaxy's disk can occur, after which the galaxy remains PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION D-8
From page 272...
... In particular, feedback affects the physical state of all baryons within a galaxy's sphere of influence and ensures that a large fraction of both gas and metals associated with the dark matter halos remain in the CGM and local IGM. Theory suggests that, independent of redshift, hot atmospheres develop only in halos ≳1012 M⊙, the expected mass threshold beyond which the supply of cool gas to the central regions of the halo is curtailed, slowing the rate of star formation.
From page 273...
... HOW DO SUPERMASSIVE BLACK HOLES FORM AND HOW IS THEIR GROWTH COUPLED TO THE EVOLUTION OF THEIR HOST GALAXIES? Investigations of SMBHs over the past ~20 years have underscored their importance for galaxy evolution but also the significant gaps that persist in our understanding of these objects.
From page 274...
... High spectral resolution X-ray kinematic measurements are a frontier scientific measurement for this question. Hitomi showed that the intracluster plasma of the Perseus cluster is PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION D-11
From page 275...
... The goal of observational and theoretical studies of the Milky Way is to understand the assembly, star formation, and chemical enrichment histories of the thin disk, thick disk, bulge, bar, and stellar halo; the origin of the striking bimodality of element abundance ratios across the disk; the importance of gas accretion, radial gas flows, fountains, and outflows through time and at the present day; the impact of dynamical perturbations on kinematic structure; the baryon content and temperature-density structure of the gaseous halo; and the mass, density profile, shape, and substructure of the dark matter halo. The final data releases from ongoing spectroscopic surveys and from the Rubin Telescope and Roman in the coming decade will greatly advance our knowledge of the structure and substructure of the Milky Way's stellar components.
From page 276...
... D-Q4b. The Threshold of Galaxy Formation The comparison between observed numbers of galaxies and predicted numbers of dark matter halos implies that the efficiency of galaxy formation plummets in halos with mass below Mhalo~109 M⊙, and observable stellar systems do not form in halos with mass Mhalo<108 M⊙.
From page 277...
... . The gain in PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION D-14
From page 278...
... from z ~ 1 to the end of reionization at z ~ 6. An important benchmark for testing the paradigm established over the past decade in which bursty star formation gravitationally heats the central regions of the host dark matter halos is to reach Mstars ≪ 109 M⊙ systems, where this feedback-induced effect is predicted to be most efficient.
From page 281...
... IFU and cycling of metals. spectroscopy, δv < 100 km/s, to map warm/hot gas (T ~ 104–107 PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION D-18
From page 282...
... How do DQ-3a. The seeds of supermassive  Multi-time domain surveys supermassive black holes black holes.
From page 283...
... Discovery Area: Mapping  Wide maps of neutral and the circumgalactic medium ionized gas emission lines of and intergalactic medium in galaxies/CGM/ICM/IGM, both emission. intensity and kinematics, out to ~0.3–1 Mpc radius, v ~ 50 km/s, at X-ray, UV, optical, IR, radio (including HI 21cm)


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