Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2023) / Chapter Skim
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Appendix D: Report of the Panel on Galaxies
Appendix D: Report of the Panel on Galaxies
Pages 271-290
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From page 271... ...
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.
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From page 272... ...
How do the histories of galaxies and their dark matter halos shape their observable properties? Discovery Area: Mapping the circumgalactic medium and the intergalactic medium in emission STATE OF THE FIELD Galaxies are open systems with extensive circulation of energy, gas, and metals between their stellar bodies and the surrounding circumgalactic medium (CGM)
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From page 273... ...
One striking puzzle is the emergence by z ~4 of massive evolved spheroids with little ongoing star formation. 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.
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From page 274... ...
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. Large populations of "ultra-diffuse galaxies," with low central surface brightness and large sizes, have been discovered and characterized in galaxy groups and clusters.
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From page 275... ...
D-Q1a. Detailed Thermal History of the Intergalactic Medium and the Topology of Reionization Intergalactic gas remains cold and opaque during the Dark Ages.
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From page 276... ...
Current expectations Population III (Pop III) stars are the first stars to form after the Big Bang, perhaps as early as z ~50–60 for Pop III stars are informed by models that include large unknowns such as their initial mass function (IMF)
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From page 277... ...
Gamma-ray bursts (GRBs) probe both early star formation, via their measured rates, and the metal enrichment of the IGM, by acting as background light sources for absorption line spectroscopy.
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From page 278... ...
of far-IR fine structure lines like [CII] is beginning to quantify the ISM in star-forming galaxies, but detailed studies of the observations at sub-kpc resolution and δv ~10–30 km/s in order to resolve and characterize the properties cold molecular gas in typical, Milky Way–like, or lower-mass, galaxies beyond z ~2 require radio-to-mm and kinematics of molecular gas clumps with masses ~ a few × 108 M⊙.
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From page 279... ...
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 quiescent until a new gas supply is accreted.
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From page 280... ...
Alternatively, distant quasars could grow from light seed BHs, such as those formed from the death of massive stars, either through suppression of feedback that modulates inflowing gas accretion rates (i.e., super-Eddington accretion) at early cosmic epochs, or through rapid merging of stellar-mass BHs accompanying hierarchical structure formation at early times.
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From page 281... ...
The highest redshifts remain largely unprobed, our knowledge of the most heavily obscured AGNs is incomplete, even at the lowest redshifts, and nuclear activity in the lowest-mass galaxies is poorly constrained. We need to fill these fundamental holes in our knowledge of AGN demographics in order to understand the mechanisms and importance of BH feedback in galaxy evolution and its interplay with star formation and star formation feedback and address deep questions about the actual physical structure of AGNs.
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From page 282... ...
Many aspects of this picture, and the progress achieved over the past ~10–20 years, have been summarized earlier in the section "State of the Field"; yet, many aspects have not been strongly tested by observations, and the discovery potential is large for many areas in the field of galaxy formation over the coming decade. In particular, a plethora of data will parse our own Milky Way into its elemental constituents, enabling us to understand its physics and how singular or general it is as a system; inroads in the investigation of the lowest-mass galaxies in the local universe will clarify the process of galaxy formation close to its mass threshold; and the characterization of the physical components of low-redshift galaxies will provide the benchmark for describing and understanding the galaxies at higher redshifts, where we do not have the luxury of high spatial resolution.
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From page 283... ...
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 present; 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 Observatory and Roman in the coming decade will greatly advance our knowledge of the structure and substructure of the Milky Way's stellar components.
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From page 284... ...
to characterize star formation histories at the earliest epochs out to ~3 Mpc. Large fields, like those covered by the Rubin Observatory and Roman, will enable efficient observations of typical Milky Way dwarfs, with high target density for simultaneous observation of nearby stars.
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From page 285... ...
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. Deep ALMA observations of the restframe far-IR [CII]
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From page 286... ...
a full baryon and metal accounting in different gas phases at different redshifts, (2) how galaxies acquire fuel for sustaining star formation, and (3)
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From page 287... ...
(d) FIGURE D.1 The circumgalactic and intergalactic medium around a simulated Milky Way progenitor at z = 2.
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From page 288... ...
SUMMARY AND FINAL CONSIDERATIONS The past two decades have firmly established the LCDM framework that sets the initial conditions for cosmological structure formation and for galaxy formation and evolution within and connected to these large-scale structures. In the coming decade, a suite of powerful facilities with unprecedented capabilities for studying galaxies will begin operations, priming the field of galaxy formation and evolution for a period of major advances.
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From page 289... ...
Detailed thermal history • Wide-field near-infrared (NIR) imaging of >105 z medium and the first sources of of the intergalactic medium and ~10–12 Mstar ~108 M⊙ galaxies, and of hundreds of z ~ radiation evolve from cosmic dawn the topology of reionization.
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From page 290... ...
Discovery Area: Mapping the • Wide maps of neutral and ionized gas circumgalactic medium and emission lines of galaxies/CGM/ICM/IGM, ~0.3–1 Mpc radius, δv ~50 km/s, at X-ray, intergalactic medium in emission. both intensity and kinematics, out to UV, optical, IR, radio (including HI 21 cm)
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