New instruments and new thinking reveal the CGM as a complex, dynamic gaseous environment that may close galactic baryon budgets and regulate gas accretion, star formation, and chemical enrichment. The observational studies that underlie the mass density profiles in Figure 7 and mass budgets in Sections 4 and 5 have all been obtained since 2010. For years, questions about how and when gaseous halos influenced galaxy evolution consistently struggled with what was there. The bulk contents of the CGM are now better characterized than ever before. There remain missing pieces — the baryon and metals budget well below L* remain to be filled in (Figure 8), and many of the metals remain missing — but we can already see signs that the most urgent questions motivating new studies take what and where as known, and go on to ask how and when. These sort of questions strike more directly at physics than at phenomenology.
9.1.1. The Scale Problem. How a gaseous halo evolves is determined at any instant primarily by its density, temperature, metallicity, and radiation fields. But for an actual CGM (such as the simulated one in Figure 6) these physical quantities vary and evolve on many relevant scales, ranging from the sub-parsec sizes for single cold clouds to the > 100 kpc size of the whole CGM and even > Mpc scales in the IGM. If we are to answer the hows of accretion, feedback, recycling, and quenching, we must achieve a better understanding of the basic physical fields at higher spatial and kinematic resolution. This means finding ways to capture sub-parsec boundary layers and instabilities while also maintaining the ≫ kpc context. Yet this 5–6 order of magnitude range still cannot be captured simultaneously in numerical simulations. One approach would be to continue the development of physically rigorous analytic models e.g., Voit et al. (2015b), Thompson et al. (2016), Fielding et al. (2016), Faerman, Sternberg & McKee (2017) that can isolate the key physical effects and then to incorporate these lessons into simulations at the subgrid level while their resolution improves with computing power. For instance, it might be possible to include subgrid models that account for unresolved interfaces between hot and cold gas, or to extract subgrid models for cosmological boxes from extremely high resolution idealized cloud simulations with carefully controlled physics. To complete the leap between phenomenology and physics, these intrinsically “sub-grid” processes must come under control while the proper cosmological and galactic context is maintained.
The transport of metals and the information they provide would also benefit from addressing the scale problem. Metals trace feedback and drive cooling, so how they are distributed through CGM gas at small scales is a critical factor in a proper physical understanding of accretion and feedback. Dense CGM gas appears bimodal in metallicity, congruent with the idea of “pristine accretion” and “recycled winds”. What does this tell us about the small-scale structure of the CGM, the relationship between accretion and feedback, and the mixing of diffuse gas? These are among the thorniest of open questions, because of the huge dynamic range in metallicity that must be captured. This problem will be addressed by larger absorber and galaxy surveys, but perhaps poses its stiffest challenges to numerical simulations, because many of the relevant physical mechanisms for mixing gas at boundaries and interfaces are still well below the “sub-grid” level of simulations. This is another case where coupling small-scale simulations of clouds to cosmological boxes could pay dividends.
The “scale” problem exists also for data but might be better labeled a problem of resolution and confusion. In data, the rich multiphase and multiscale structures of CGM gas are seen through a complex rendering in absorption or emission lines from diagnostic ions. The line profiles of absorbers likely contain more information than we are currently able to extract and interpret. Systematic effects from line saturation, uncertain ionization and radiation fields, relative abundances, limited signal-to-noise, and finite spectral resolution all complicate the derivation of the true CGM density field, which in turn enters into mass estimates, energy balance, and timescales for the gas flows of interest. While we are learning to model and simulate the CGM at higher resolution with better physics, we should also aim to extract and use the full information available in the rich kinematic profiles of multiphase absorbers, which will likely require new analytic and statistical techniques. The importance and complexity of the CGM make it imperative to examine all of the information that Nature provides.
9.1.2. Mass Flows and the Fate Problem. The CGM matters to galaxies as long as it provides them with fuel and recycles their feedback. Ultimately this is what we care about most — how does the CGM influence galaxy evolution? The most fundamental questions with which we began are still not completely answered: How does cold gas accrete and form stars over billions of years, and why does this cycle stop in massive galaxies? Does the CGM empty out or get consumed when galaxies quench? How much star formation is fueled by recycling and how much by new accretion? Can we ever hope to identify particular absorbers as accretion, feedback, or recycling, or are we destined never to separate them? These questions will drive the field as it advances from phenomenology toward more sophisticated physical understanding. Properly explaining these phenomena in terms of the hows of accretion, feedback, recycling, and quenching requires that we follow mass flows, not merely mass budgets.
Now that we have a grip on the bulk contents of the CGM, it is time to develop and deploy the tools to probe these questions of how the gas flows operate. To follow flows, we will need to make at least three key advances. First, the mass budgets should be characterized more fully in all phases at stages of galaxy evolution, including those that are relatively short lived such as mergers and AGN. These analyses would additionally benefit from analyzing how outflows and inflows seen in down-the-barrel measurements relate to the kinematics viewed on transverse sightlines, an overdue synthesis deserving attention from both observations and theory. Second, we must attempt to directly constrain the timescales of CGM evolution using data alone — how do mass budgets and kinematics jointly constrain timescales? Third, we must look at simulations in a new way that focuses on the origins and evolution of the physical phases and how these appear in the data. A large measure of simulation work addressed to the CGM has focused on using column densities and kinematics to constrain uncertain mechanisms of feedback by matching real data to mocks from simulations. While these issues are being resolved, it is also valuable to look at simulations from a different phenomenological point of view. The study from Ford et al. (2014) provides an example; that paper identified particles as “pristine accretion”, “recycled accretion”, “young outflows”, and “ancient outflows” and followed their evolution over time. These insightful categories turn out to be correlated with observable signatures. We believe there is great potential in viewing models and data from this angle, trying to identify the more distinctive or even unique manifestations of key physical processes defined by their “fate” rather than their instantaneous properties or appearance.
9.2. Future Prospects for Data
The next decade should bring a wide array of new instruments and numerical capabilities that will address these unsolved problems.
While Hubble lasts (mid-2020s), UV absorber samples will grow, particularly those that focus on the z > 0.5 regime where a broader set of EUV ionization diagnostics is available (such as Ne viii). This increase in coverage will in turn allow more careful treatments of ionization diagnostics component-by-component, hopefully with a better understanding of how CGM gas is spread across physical phases and across galaxy mass. COS remains the ideal instrument for this problem, and big advances are still possible in the metals budget, ionization and kinematic relationships of multiphase gas, and the relationships between CGM gas and special types of galaxies. Starting in 2018, the James Webb Space Telescope (JWST) will enable much deeper searches for faint galaxies near QSO sightlines, likely associating galaxies with samples of z > 4 absorbers that are already known (Becker, Bolton & Lidz, 2015, Matejek & Simcoe, 2012). Detections of H i emission (e.g., Martin et al., 2015, Arrigoni Battaia et al., 2015, Cantalupo et al., 2014) will provide useful tests of models for CGM mass and structure, but the problems of gas ionization state and metal transport will require much more challenging maps of emission from oxygen and carbon ions (see Hayes et al., 2016, for a pioneering effort). Such maps might emerge from IFU spectrographs such as MUSE and KCWI, and their successors on 30m class telescopes; limits can be further improved by stacking of multiple galaxies. The optimal galaxies would be those where absorption line probes are also available, so that emission-line and pencil-beam measurements can be compared. Emission maps of metal-bearing CGM gas (e.g. Bertone et al., 2010, Corlies & Schiminovich, 2016) are a key goal of the Large Ultraviolet/Optical/Near Infrared Surveyor (LUVOIR 3), which will push to 50x the UV point source sensitivity of Hubble/COS and 100-fold multiplexing in UV spectroscopy. Planned for launch in the 2030s, LUVOIR would be able to directly image the CGM in metal-line emission, map the most diffuse gas with weak absorbers, and resolve the multiphase kinematics of CGM gas with R > 50 000 UV spectroscopy (Dalcanton et al., 2015). The hot gas phase would be addressed by the ESA-planned X-ray flagship known as the Advanced Telescope for High ENergy Astrophysics (ATHENA 4) in 2028, with a significant focus on understanding the cosmic evolution of hot gas in the IGM and CGM.
The size of our samples provide statistical power over the key galaxy variables: mass, redshift, shape, evolutionary state, and orientation to the sightline. Here, future UV absorber samples must be supplemented by optical absorber samples at z ∼ 3, and by deeper galaxy surveys at all redshifts. This is a problem for the next generation of giant ground-based telescopes, which will advance high-z CGM studies in rest-UV lines and support low-z studies by obtaining redshifts of sub-L* galaxies near QSO sightlines at surveys at z < 1 to fill in the low-mass baryon and metals census, still a major missing piece.
Massive fiber based surveys have proven effective at characterizing CGM gas and its flows with both intervening and down the-barrel measurements. This technique should only accelerate in the future, pushing to fainter sources, higher redshifts, and rarer foreground galaxies with future massively multiplexed spectrographs (e.g., eBOSS, PFS) on large telescopes. This technique excels at detecting weak signals in the CGM, and at examining more and more foreground galaxy properties with good statistics. With larger, deeper samples, we can look forward to addressing questions about the behavior of the cold/dense CGM in rarer galaxy types, such as quasars and AGN, mergers, and groups.
Galaxies were understood as island universes long before astronomers discovered the interstellar gas that forms their stars. The intergalactic medium was added to the big picture with the discovery of QSO absorption lines and the development of the dark-matter cosmology. Because it is much fainter than stars, and much smaller than the IGM, the CGM is arguably the last major component of galaxies to be added but it has nevertheless become a vital frontier. As to why, it is clear that much has been learned by viewing galaxy evolution from the perspective of the CGM. The circumgalactic medium can even provoke fascination: might the heavy elements on Earth cycled back and forth through the Milky Way's CGM multiple times before the formation of the Solar System? It appears that the solution to major problems in galaxy formation that are still unsolved will run through this elusive region of the cosmos.
DISCLOSURE STATEMENT
The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.
ACKNOWLEDGMENTS
MSP and JT acknowledge support from NSF grant AST-1517908. We are grateful to Ann Feild of STScI for her expert artistic contributions, to Joop Schaye and Ben Oppenheimer for use of the EAGLE simulation shown in Figures 2, 6, and 8, to Josh Suresh for data from the Illustris simulation shown in Figure 8, to Sasha Muratov for data from the FIRE simulation (Figure 8), and to Ben Oppenheimer for the data from the specially-analyzed EAGLE halos shown in Figure 9. We also thank Lauren Corlies, Matt McQuinn, Andrew Fox, Romeel Davé, and John O’Meara for insightful comments on a draft of this article. We have made extensive use of NASA's Astrophysics Data System, astropy (Robitaille et al., 2013), matplotlib (Hunter, 2007), yt (Turk et al., 2011), and the python tools Colossus from Benedikt Diemer and Seaborn by Michael Waskom.
3 http://asd.gsfc.nasa.gov/luvoir/ Back.
4 http://sci.esa.int/cosmic-vision/54517-athena/ Back.