Spectroscopy of galaxy continua has been used for decades as a powerful probe of the kinematics of gas in the foreground. Absorption transitions sensitive to cool, diffuse material trace its motion with respect to the galaxy's stellar component along the line of sight. The galaxy continuum “beam” is absorbed by gas over a broad range of scales, from the ∼ 100−200 pc scale heights of the dense interstellar medium (Langer et al. 2014) to the > 100 kpc extent of the diffuse gas reservoir filling the galaxy halo (Prochaska et al. 2011, Tumlinson et al. 2011, Werk et al. 2014) and beyond.
The detection of absorption lines in a galaxy's spectrum (i.e., “down the barrel”) which are redshifted with respect to its rest frame is unequivocal evidence of gas flow toward the continuum source. And while this signature arises from material at any location along the sightline to the galaxy, such that the flow may not ultimately reach the galaxy itself, redshifted absorption has been broadly interpreted as strong evidence for gas inflow toward or accretion onto the background host. Over the past ten years, as high signal-to-noise (S/N) galaxy spectroscopy covering the rest-frame optical into the near-ultraviolet has become more routine, this technique has become sensitive to the inflow of material over a broad range of densities and temperatures: redshifted Ca II H & K λ λ 3934, 3969 or Na I D λ λ 5891, 5897 absorption probes cold, mostly neutral gas infall at a temperature T < 1000 K; redshifts in low-ionization transitions such as Mg II λ λ 2796, 2803 or Fe II λ λ 2586, 2600 trace the inflow of cool, photoionized gas at T ∼ 104 K; and the detection of redshifted C IV λ λ 1548, 1550 or Si IV 1394, 1402 absorption would in principle trace yet warmer (T ∼ 105 K) gas accretion.
Furthermore, unlike background QSO absorption line experiments, which typically must adopt the assumption that inflowing gas has a relatively low metallicity (Z / Z⊙ < 1; Lehner et al. 2013) in order to disentangle accreting systems from those enriched by galactic outflows, redshifted self-absorption naturally traces metal-rich inflow, and may even trace pristine inflow in rare cases where spectral coverage of Lyα is available (Fathivavsari et al. 2016). Moreover, while studies searching for the signature of gas accretion in emission from the 21cm transition of neutral hydrogen are currently limited by the faint surface brightness of such features to galaxies within a few hundred Mpc of our own (Martin et al. 2010), the “down the barrel” technique has been used to detect gas accretion onto galaxies as distant as z ∼ 1 (Coil et al. 2011, Rubin et al. 2012, Martin et al. 2012).
In spite of these clear advantages, however, the first report of redshifted absorption observed down the barrel toward a sample of more than a single object did not occur until 2009 (Sato et al. 2009). Even today, secure detections of this phenomenon have been reported for only ∼ 60−80 systems. Instead, measurements of blueshifted absorption tracing cool gas outflow have dominated the literature (Heckman et al. 2000, Martin 2005, Rupke et al. 2005b, Weiner et al. 2009, Steidel et al. 2010, Chen et al. 2010b). Large-scale galactic winds, thought to be driven by processes associated with star formation, are now known to arise ubiquitously in star-forming objects from the local universe to z > 2 (Heckman et al. 2000, Ajiki et al. 2002, Martin et al. 2012, Rubin et al. 2014). These winds are likewise traced by all of the metal-line absorption features listed above, and are observed to have velocities ranging from ∼ −50 km s−1 to < −800 km s−1 (Steidel et al. 2010, Rubin et al. 2014, Du et al. 2016). As the free-fall velocity of material within the virial radius of a typical massive, star-forming galaxy halo at z ∼ 0 (with halo mass Mh ∼ 1011−12 M⊙) is expected to be only ∼ 100−200 km s−1 (Goerdt & Ceverino 2015), the preponderance of winds covering the sightlines to galaxies must necessarily obscure the detection of material falling inward at such comparatively modest velocities. Indeed, this issue is compounded by the low spectral resolution of the vast majority of spectroscopic surveys useful for these analyses (Weiner et al. 2009, Steidel et al. 2010, Martin et al. 2012, Rubin et al. 2014).
In this chapter, we review the works in which the few bona fide instances of gas inflow were reported, beginning with the first detections via transitions in the rest-frame optical in Section 2. Due to technical limitations (described below), the focus of these early studies was on red, early-type galaxies and/or galaxies exhibiting signs of AGN activity. Reports of inflow onto galaxies hosting the most luminous AGN (i.e., bright QSOs) are discussed in Section 2.3. In Section 3, we describe the first detections of inflow onto actively star-forming systems facilitated by high-S/N spectroscopic galaxy surveys in the rest-frame ultraviolet. The biases inherent in the use of the “down the barrel” technique given the ubiquity of galactic winds are described in Section 4.1, and unique constraints on the morphology of gas inflow which will soon be facilitated by ongoing spatially-resolved spectroscopic surveys are discussed in Section 4.2. Section 5 presents a summary, and offers some recommendations for future experiments which will leverage the full potential of this technique in the detection and characterization of the process of gas accretion onto galaxies.