Absorption-line spectroscopy complements emission surveys and provides a powerful tool for studying the diffuse, large-scale baryonic structures in the distant Universe (e.g., Rauch 1998, Wolfe et al 2005, Prochaska and Tumlinson 2009). Depending on the physical conditions of the gas (including gas density, temperature, ionization state, and metallicity), a high-density region in the foreground is expected to imprint various absorption transitions of different line strengths in the spectrum of a background QSO. Observing the absorption features imprinted in QSO spectra enables a uniform survey of diffuse gas in and around galaxies, as well as detailed studies of the physical conditions of the gas at redshifts as high as the background sources can be observed.
Figure 1 displays an example of optical and near-infrared spectra of a high-redshift QSO. The QSO is at redshift zQSO = 4.13, and the spectra are retrieved from the XQ-100 archive (Lopez et al 2016). At the QSO redshift, multiple broad emission lines are observed, including the Lyα/N V emission at ≈ 6200 Å, C IV emission at ≈ 7900 Å, and C III] emission at ≈ 9800 Å. Blueward of the Lyα emission line are a forest of Lyα λ 1215 absorption lines produced by intervening overdense regions at zabs ≲ zQSO along the QSO sightline. These overdense regions span a wide range in H I column density (N(H I)), from neutral interstellar gas of N(H I) ≥ 1020.3 cm−2, to optically opaque Lyman limit systems (LLS) of N(H I) > 1017.2 cm−2, to optically thin partial LLS (pLLS) with N(H I) = 1015−17.2 cm−2, and to highly ionized Lyα forest lines with N(H I) = 1012−15 cm−2 (right panel of Fig. 2).
Figure 1. Example of the wealth of information for intervening gas revealed in the optical and near-infrared spectrum of a QSO at z = 4.13. In addition to broad emission lines intrinsic to the QSO, such as Lyα/N V at ≈ 6200 Å, a forest of Lyα λ 1215 absorption linesLyman α forest is observed blueward of 6200 Å. These Lyα forest lines arise in relatively high gas density regions at zabs ≲ zQSO along the line of sight. The Lyα absorbers span over 10 decades in neutral hydrogen column densities (N(H I)), and include (1) neutral damped Lyα absorbers (DLAs), (2) optically thick Lyman limit systems (LLS), (3) partial LLS (pLLS), and (4) highly ionized Lyα absorbers (see text for a quantitative definition of these different classes). The DLAs are characterized by pronounced damping wings (second panel from the top), while LLS and pLLS are identified based on the apparent flux discontinuities in QSO spectra (top panel). Many of these strong Lyα absorbers are accompanied with metal absorption transitions such as the O VI λλ 1031, 1037 doublet transitions which occur in the Lyα forest, and the C IV λλ 1548, 1550 and Mg II λλ 2796, 2803 doublets. Together, these metal lines constrain the ionization state and chemical enrichment of the gas.
The large N(H I) in the neutral medium produces pronounced damping wings in the QSO spectrum. These absorbers are commonly referred to as damped Lyα absorbers (DLAs). An example is shown in the second panel from the top in Fig. 1. In this particular case, a simultaneous fit to the QSO continuum and the damping wings (red curve in the second panel from the top) yields a best-fit log N(H I) = 21.45 ± 0.05 for the DLA. At intermediate N(H I), LLS and pLLS are identified based on the apparent flux discontinuities in QSO spectra (top panel). A significant fraction of these strong Lyα absorbers have been enriched with heavy elements which produce additional absorption features due to heavy ions in the QSO spectra. The most prominent features include the O VI λλ 1031, 1037 doublet transitions which occur in the Lyα forest, and the C IV λλ 1548, 1550 and Mg II λλ 2796, 2803 doublets, plus a series of low-ionization transitions such as C II, Si II, and Fe II. Together, these ionic transitions constrain the ionization state and chemical compositions of the gas (e.g., Chen and Prochaska 2000, Werk et al 2014).
Combining galaxy surveys with absorption-line observations of gas around galaxies enables comprehensive studies of baryon cycles between star-forming regions and low-density gas over cosmic time. At low redshifts, z ≲ 0.2, deep 21 cm and CO surveys have revealed exquisite details of the cold gas content (T ≲ 1000 K) in nearby galaxies, providing both new clues and puzzles in the overall understanding of galaxy formation and evolution. These include extended H I disks around blue star-forming galaxies with the H I extent ≈ 2 × what is found for the stellar disk (e.g., Swaters et al 2002, Walter et al 2008, Leroy et al 2008), extended H I and molecular gas in early-type galaxies (e.g., Oosterloo et al 2010, Serra et al 2012) with predominantly old stellar populations and little or no on-going star formation (Salim and Rich 2010), and widespread H I streams connecting regular-looking galaxies in group environments (e.g., Verdes-Montenegro et al 2001, Chynoweth et al 2008).
Figure 2 (left panel) showcases an example of a deep 21 cm image of the M81 group, a poor group of dynamical mass Mdyn ∼ 1012 M⊙ (Karachentsev and Kashibadze 2006). Prominent group members include the grand-design spiral galaxy M81 at the centre, the proto-starburst galaxy M82, and several other lower-mass satellites (Burbidge and Burbidge 1961). The 21 cm image displays a diverse array of gaseous structures in the M81 group, from extended rotating disks, warps, high velocity clouds (HVCs), tidal tails and filaments, to bridges connecting what appear to be optically isolated galaxies. High column density gaseous streams of N(H I) ≳ 1018 cm−2 are seen extending beyond 50 kpc in projected distance from M81, despite the isolated appearances of M81 and other group members in optical images. These spatially resolved imaging observations of different gaseous components serve as important tests for theoretical models of galaxy formation and evolution (e.g., Agertz et al 2009, Marasco et al 2016). However, 21 cm imaging observations are insensitive to warm ionized gas of T ∼ 104 K and become extremely challenging for galaxies beyond redshift z = 0.2 (e.g., Verheijen et al 2007, Fernández et al 2013).
Figure 2. Mapping galaxy outskirts in 21 cm and in QSO absorption-line systems. Left: Deep 21 cm image of the M81 group, revealing a complex interface between stars and gas in the group. The observed neutral hydrogen column densities range from N(H I) ∼ 1018 cm−2 in the filamentary structures to N(H I) > 1021 cm−2 in the star-forming disks of group members (Yun et al 1994, Chynoweth et al 2008). The 21 cm image reveals a diverse array of gaseous structures in this galaxy group, but these observations become extremely challenging beyond redshift z ≈ 0.2 (e.g., Verheijen et al 2007, Fernández et al 2013). Right: The H I column density distribution function column density distribution function of Lyα absorbers, fN(H I), uncovered at z = 1.9 − 3.2 along sightlines toward random background QSOs (adapted from Kim et al 2013). Quasar absorbers in different categories are mapped onto different H I structures both seen and missed in the 21 cm image in the left panel. Specifically, DLAs probe the star-forming ISM and extended rotating disks, LLS probe the gaseous streams connecting different group members as well as stripped gas and high velocity clouds around galaxies, and pLLS and strong Lyα absorbers trace ionized gas that is not observed in 21 cm signals. Among the quasar absorbers, C IV absorption transitions are commonly observed in strong Lyα absorbers of N(H I) ≳ 1015 cm−2 (see e.g., Kim et al 2013, D'Odorico et al 2016), and Mg II absorption transitions are seen in most high-N(H I) absorbers of N(H I) ≳ 1016 cm−2 (see e.g., Rigby et al 2002). These metal-line absorbers trace chemically enriched gas in and around galaxies.
QSO absorption spectroscopy extends 21 cm maps of gaseous structures around galaxies to both lower gas column density and higher redshifts. Based on the characteristic N(H I), direct analogues can be drawn between different types of QSO absorbers and different gaseous components seen in deep 21 cm images of nearby galaxies. For example, DLAs probe the neutral gas in the interstellar medium (ISM) and extended rotating disks, LLS probe optically thick gaseous streams and high velocity clouds in galaxy haloes, and pLLS and strong Lyα absorbers of N(H I) ≈ 1014−17 cm−2 trace ionized halo gas and starburst outflows (e.g., supergalactic winds in M82 Lehnert et al 1999) that cannot be reached with 21 cm observations.
The right panel of Fig. 2 displays the H I column density distribution function, fN(H I), for all Lyα absorbers uncovered at z = 1.9 − 3.2 along random QSO sightlines (Kim et al 2013). fN(H I), defined as the number of Lyα absorbers per unit absorption pathlength per unit H I column density interval, is a key statistical measure of the Lyα absorber population. It represents a cross-section weighted surface density profile of hydrogen gas in a cosmological volume. With sufficiently high spectral resolution and high signal-to-noise, S/N ≳ 30, QSO absorption spectra probe tenuous gas with N(H I) as low as N(H I) ∼ 1012 cm−2. The steeply declining fN(H I) with increasing N(H I) shows that the occurrence (or areal coverage) of pLLS and strong Lyα absorbers of N(H I) ≈ 1014−17 cm−2 is ≈ 10 times higher than that of optically thick LLS along a random sightline and ≈ 100 times higher than the incidence of DLAs. Such a differential frequency distribution is qualitatively consistent with the spatial distribution of H I gas recorded in local 21 cm surveys (e.g., Fig. 2, left panel), where gaseous disks with N(H I) comparable to DLAs cover a much smaller area on the sky than streams and HVCs with N(H I) comparable to LLS. If a substantial fraction of optically thin absorbers originate in galaxy haloes, then their higher incidence implies a gaseous halo of size at least three times what is seen in deep 21 cm images.
In addition, many of these strong Lyα absorbers exhibit associated transitions due to heavy ions. In particular, C IV absorption transitions are commonly observed in strong Lyα absorbers of N(H I) ≳ 1015 cm−2 (see, e.g., Kim et al 2013, D'Odorico et al 2016), and Mg II absorption transitions are seen in most high-N(H I) absorbers of N(H I) ≳ 1016 cm−2 (e.g., Rigby et al 2002). While Mg II absorbers are understood to originate in photo-ionized gas of temperature T ∼ 104 K (e.g., Bergeron and Stasińska 1986), C IV absorbers are more commonly seen in complex, multi-phase media (e.g., Rauch et al 1996, Boksenberg and Sargent 2015). These metal-line absorbers therefore offer additional probes of chemically enriched gas in and around galaxies.
This Chapter presents a brief review of the current state of knowledge on the outskirts of distant galaxies from absorption-line studies. The review will first focus on the properties of the neutral gas reservoir probed by DLAs, and then outline the insights into star formation and chemical enrichment in the outskirts of distant galaxies from searches of DLA galaxies. A comprehensive review of DLAs is already available in Wolfe et al (2005). Therefore, the emphasis here focusses on new findings over the past decade. Finally, a brief discussion will be presented on the empirical properties and physical understandings of the ionized circumgalactic gas as probed by strong Lyα and various metal-line absorbers.