In recent years, there has been a growing realization that the “cosmic baryon cycle” is both a primary driver and a primary regulator of galaxy formation. Continuous accretion of gas from the intergalactic medium (IGM) is necessary to sustain observed star formation rates (SFRs) over a Hubble time (e.g., Erb, 2008; Prochaska & Wolfe, 2009; Bauermeister et al., 2010). However, models in which the intergalactic gas accreted by galaxies is efficiently converted into stars produce galaxies with stellar masses that exceed observed ones by an order of magnitude or more (e.g., White & Frenk, 1991; Navarro et al., 1995; Kereš et al., 2009a). In the latest generation of models, star formation-driven galactic winds regulate galaxy growth below ∼ L⋆ by ejecting back into the IGM most of the accreted gas before is has time to turn into stars (see the review by Somerville & Davé, 2015). Despite a broad consensus regarding the importance of inflows and outflows in galaxy evolution, many questions regarding their nature and effects remain at the forefront of current research.
For example, many cosmological simulations and semi-analytic models now suggest that wind recycling (the fallback of gas previously ejected in galactic winds) plays an important role in shaping the galaxy stellar mass function and setting the level of late-time galactic accretion (Oppenheimer et al., 2010; Henriques et al., 2013; Anglés-Alcàzar et al., 2016). While galactic accretion is a generic prediction of cosmological simulations (e.g., Kereš et al., 2005; Kereš et al., 2009b; Dekel et al., 2009; Brooks et al., 2009; Faucher-Giguère et al., 2011; van de Voort et al., 2011), its properties are subject to uncertainties in how the accretion flows are affected by shocks and hydrodynamical instabilities as they interact with galaxy halos (e.g., Birnboim & Dekel, 2003; Nelson et al., 2013; Mandelker et al., 2016). Galactic winds are driven by feedback processes that operate on the scale of individual star-forming regions, which are generally not well resolved in current simulations. As a result, detailed properties such as their phase structure remain uncertain even in today's highest resolution zoom-in simulations of galaxy formation (e.g., Shen et al., 2013; Hopkins et al., 2014; Marinacci et al., 2014; Agertz & Kravtsov, 2015). In large-volume cosmological simulations, it is not yet possible to resolve how galactic winds are launched so even the bulk properties of galactic winds in such simulations are typically tuned to match observables such as the galaxy stellar mass function (e.g., Davé et al., 2011; Vogelsberger et al., 2014; Schaye et al., 2015). Theoretical predictions for inflows and outflows are furthermore complicated by the fact that inflows and outflows inevitably interact with each other (e.g., van de Voort et al., 2011; Faucher-Giguère et al., 2011; Faucher-Giguère et al., 2015; Nelson et al., 2015).
The importance of inflows and outflows for galaxy evolution, as well as the significant theoretical uncertainties, imply that observations of these processes are critical to test and inform galaxy formation theories. Since observational techniques for probing inflows and outflows generally provide only fragmentary information about the physical nature of the observed gas (e.g., 1D skewers through galactic halos for typical quasar absorption line measurements), forward modeling using cosmological simulations and comparing the simulations with observations will likely continue to play a central role in disentangling these processes. In this chapter, we review the current status of using cosmological simulations to develop observational diagnostics of galactic accretion. Since the dynamics inflows and outflows are intertwined in the circum-galactic medium (CGM), this chapter will also cover relevant outflow diagnostics.
This chapter is largely organized around our group's research on the topic, but attempts to provide a broad review of theoretical research relevant to interpreting recent and upcoming observations. The chapter is divided into two main sections, one on absorption diagnostics (§ 2) and one on emission diagnostics (§ 3). Interspersed within our discussion of different observational diagnostics, we include some remarks on numerical uncertainties and the sensitivity of different predictions to the numerical method employed. We conclude in § 4 with a synthesis of lessons from existing simulations of galactic accretion and comparisons with observations, and suggest some promising directions for future work. We focus on observational diagnostics applicable to galaxies other than the Milky Way.