1. A VERY BRIEF HISTORY

In the mid-1950s, Guido Münch observed neutral sodium (Na i) and singly-ionized calcium absorption (Ca ii) in the spectra of hot stars at high Galactic latitudes. Before these data were published as Münch & Zirin (1961), Münch showed them to Lyman Spitzer, who interpreted the lines as evidence for diffuse, extraplanar hot gas (T ∼ 10⁶ K), which keeps the colder clouds traced by Na i and Ca ii in pressure confinement (Spitzer, 1956). And so was born the idea of the “Galactic corona” and its exploration by absorption lines in the spectra of background objects. Following Schmidt's 1963 discovery of quasars, studies of “extragalactic” gas rapidly progressed with spectroscopy of the intervening absorption lines by J. Bahcall, M. Burbidge, J. Greenstein, W. Sargent, and others. Bahcall & Spitzer (1969) then proposed that “most of the absorption lines observed in quasi-stellar sources with multiple absorption redshifts are caused by gas in extended halos of normal galaxies”. In the 1980s, subsets of the QSO absorption lines were associated with galaxies (Bergeron, 1986, Bergeron & Boissé, 1991) while the "Lyman alpha forest" emerged as their IGM counterpart (Sargent et al., 1980). Spurred by these developments, Hubble and Keck made great leaps in the 1990s towards a broader characterization of the number density and column density distribution of the IGM and CGM back to z ∼ 3. Pioneering studies from Hubble's Key Project on QSO absorption lines demonstrated that galaxy halos give rise to strong Lyα, C iv, and other metal lines e.g. Lanzetta et al. (1995), Chen et al. (1998) in a gaseous medium that is richly structured in density, temperature, and ionization (Figure 1).

Figure 1. A cartoon view of the CGM. The galaxy's red central bulge and blue gaseous disk are fed by filamentary accretion from the IGM (blue). Outflows emerge from the disk in pink and orange, while gas that was previously ejected is recycling. The “diffuse gas” halo in varying tones of purple includes gas that is likely contributed by all these sources and mixed together over time.

In the 2000s, large galaxy surveys such as SDSS uncovered the galactic baryon deficit, the mass metallicity relation, and quenching problems (Section 2). Meanwhile theorists implemented new physical prescriptions for gas accretion and feedback with new numerical methods and faster computers. It soon became impossible even to address these big mysteries of galaxies without appealing to gas flows between the ISM, the IGM, and by implication, the CGM. Yet most such models of gas flows were, and are still, tested against observations of starlight — the same observations that first posed the problems. By the mid-2000s, models and observations of gas flows in and out of galaxies had reached the point that the former were in urgent need of direct observations of the gas flows themselves. CGM studies leaped forward in the late 2000s with the installation of Hubble's Cosmic Origins Spectrograph, which was designed for reaching diffuse gas with 30 × the sensitivity of its predecessors, and with new techniques for stacking and combining X-ray and optical spectra. This, then, is the context in which our review occurs. We aim to survey recent progress in observing and modeling the gas flows that drive galaxy evolution and thus to tell the story of galaxy evolution writ large, from the perspective of the CGM.

For additional perspective on the issues raised here from a more Galactic point of view, we recommend the recent Annual Review on halo gas by Putman, Peek & Joung (2012b). For an up-to-date survey of accretion, see the forthcoming volume “Gas Accretion onto Galaxies” (Fox & Davé, 2017).