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The 1960s through the 1970s was an exciting era of the discovery of quasars. During this time the study of these cosmologically distant luminous sources developed into a powerful tool that changed the course of the science of astronomy.  This story runs in parallel with technological advances in both light-gathering capability and computing power.  In this chapter, we chart the development of the study of quasars and show how quasar absorption lines provide a tool for studying the properties of diffuse gas across the full dynamic range of astrophysical environment out to the highest redshifts

Quasar absorption line studies have matured into a modern science that has contributed to the development of our contemporary cosmological paradigm, ranging from the Big Bang, across Cosmic Noon, to the Present Epoch.  Researchers focus on key ions, transitions, and absorption lines because they are most common in the Universe.  Each of these lines has a unique cosmic visibility in that there is a strong relationship between the observed wavelength of a redshifted line, the cosmic era in which it originated, and the type of astrophysical environment it probes.  In this chapter, we outline the main eras of the evolution of the Universe, describe the phases and ionization conditions of the gas in the Universe, and show the connection between ions/transitions and the cosmic era and gas phases they probe.

Each quasar sightline provides only a single ``pencil beam” core sample of the Universe. Quasar absorption line surveys that employ numerous quasar sightlines aim to measure several key quantities (known as absorber population statistics) that characterize the astrophysical properties of absorbers. Creative methods and experimental approaches have been developed over the last decades. These include rudimentary tomography experiments using close groupings of multiple quasars for measuring transverse properties of absorbers, stacking of spectra, peering “down-the-barrel,” and the use of Gamma-ray bursts. In this chapter, we describe these ``key” population statistics and how they are measured in principle. We also describe the innovative ways that multiple sightlines are used and other experimental modern techniques.  

Hydrogen is the most abundant element in the Universe and neutral hydrogen, HI, is present in virtually all astrophysical structures ranging from the filamentary cosmic web to the inner regions of galaxies to the intracluster medium.  The absorption transition from ground state to the lowest excitation state in neutral hydrogen gives rise to the countless optically thin Lya forest lines and, in the highest column density structures, the damped Lya absorption lines (DLAs). In optically thick structures, radiative ionization creates sharp ``breaks” in quasar spectra called Lyman-limit systems (LLSs). HI correlates with the overdensity of the astrophysical environment, but this relationship evolves with redshift. HI also traces the mass density of neutral gas and the ionization history of the Universe.  In this chapter, we describe the cosmic evolution of Lya absorbers as recorded in quasar spectra from the Epoch of Reionization to the present epoch.  At the highest redshifts, the transition from a dense Lya forest to Lya spikes to the famous Gunn-Peterson trough is described.

The abundance of deuterium, an isotope of hydrogen, is sensitive to the ratio of the cosmic baryon density to the photon density. This ratio is fixed (or “frozen in”) well before Big Bang nucleosynthesis begins. Competing with the timescale over which fusion is building up helium-4 nuclei is the timescale for the photodissociation of deuterium and the b-decay rate of free neutrons. Together, these form the highly sensitive “deuterium bottleneck.” In the 1990s, measurements of the cosmic deuterium abundance using quasar absorption line techniques varied by an order of magnitude. After 25 years of effort, the scatter has been reduced to sub-1% precision and the highly sought cosmic D/H ratio was pinned down. Additional constraints are obtained using the cosmic microwave background, but these can be in tension with quasar absorption line results. In this chapter, we describe efforts to measure the cosmic deuterium abundance and reconcile them with theoretical predictions, which may be limited by the accuracy of the reaction rates used for Big Bang Nucleosynthesis calculations.

The spin-flip of the ground-state electron in neutral hydrogen, known as hyperfine structure, gives rise to the famous 21-cm line in the radio band.  The 21-cm line informs us about the “cold'' phase of atomic hydrogen in the Universe.  In this chapter, we present the basic physics of the 21-cm spin flip and then discuss several surveys of 21-cm absorption and their reported findings. These include blind surveys, galaxy-selected surveys, metal-line selected surveys, and DLA-selected surveys. An exciting measurement known as redshift drift, which would provide a direct measurement of the change in the expansion rate of the Universe, is expected to be highly precise using 21-cm observations.  During the Epoch of Reionization, a 21-cm forest, analogous to the Lya forest, is expected. In fact, this absorption line is expected to trace all the way back into the Dark Ages of the universe and yield empirical insights into the formation of the first stars and black holes.

Helium is the second most abundant element in the universe, and, when singly ionized, is hydrogenic. This means HeII has a hydrogen-like absorption spectrum but with transition energies a factor of 4 higher. This places HeII Lya forest lines deep into the ultraviolet, the consequences of which highly limit the redshift visibility of HeII studies --- only favorable quasar sightlines can be used to study HeII Lya and Lyb absorption. The column density ratio of HeII to HI is highly sensitive to the shape and intensity of the cosmic ultraviolet background (UVB), and thus is a key quantity for constraining the evolution and patchiness of the UVB. An Epoch of HeII Reionization stretching into the Cosmic Noon era provides insights into the appearance of the first quasars in the Universe.  In this chapter, we describe the redshift visibility of HeII absorbers, discuss the cosmic impact of HeII absorption, and describe key observational results, including the so-called hardness parameter, the HeII Gunn-Peterson trough, and HeII Lya spikes.

Studies of the low-ionization metal-line absorbers provide insights into cool/warm higher-density gas that has been processed through stars in galaxies. These absorbers have been studied primarily using the abundant neutral atoms sodium, oxygen, and carbon (NaI, OI, and CI), as well as the singly ionized ions of carbon, silicon, calcium, and magnesium (CII, SiII, CaII, and MgII). For optical quasar spectroscopy, these ions have limited visibilities over different redshift ranges. The advent of sensitive UV and IR spectrographs expanded the redshift coverage of MgII absorbers from z=0 to z=7. However, the redshift visibility of OI, CI, CII, and SiII remain limited because of far-ultraviolet transitions. The population statistics measured include the redshift path density, the equivalent width and column density distributions, the cosmic mass densities, and the kinematics (broadening parameters, velocity splitting distributions, and absorber velocity widths). In this chapter, we discuss multiple observational programs and their reported findings for several of the ions.

Studies of the intermediate-ionization metal-line absorbers provide insights into warm/hot lower-density gas that has been processed through stars in galaxies. These absorbers have been studied primarily using doubly and triply ionized carbon and silicon ions (CIII, CIV, SiIII, and SiIV). CIII arises deep within the spectral range of the Lya forest and is thus mostly visible at low redshifts where the Lya forest line density is much smaller. SiIII is adjacent to the Lya line and is also best surveyed at low redshift. The CIV and SiIV lines are well redward of the Lya line and thus have visibility over a wide range of redshift.  UV and IR spectrographs expanded the redshift coverage from z=0 to z=7. The population statistics measured include the redshift path density, the equivalent width and column density distributions, the cosmic mass densities, and the kinematics (broadening parameters, velocity splitting distributions, and absorber velocity widths). In this chapter, we discuss multiple observational programs and their reported findings for several of these ions.

In the 1950s, Lyman Spitzer predicted that a hot gaseous medium surrounded the Milky Way in a halo/corona and that this gas should be detectable in strong absorption from highly ionized oxygen and nitrogen. It was confirmed in the 1970s using the Copernicus satellite. In the early-1990s, the first hydrodynamic cosmological simulations predicted that a warm-hot Intergalactic medium (WHIM) was pervasive and extended out to the mildly overdense regions in the Universe. At low redshifts, the WHIM was predicted to harbor most of the baryons in the Universe. This was a bold prediction in which five-, six-, and seven-times ionized oxygen (OVI, OVII, and OVIII) was predicted to trace this gas in absorption. The latter two require X-ray spectroscopy, which has its challenges. The WHIM is also believed to be the source of the so-called broad Lya absorbers (BLAs) in the Lya forest. The WHIM can also be probed using fast radio bursts. In this chapter, we describe the discovery and confirmation of the WHIM and its characteristic properties. This includes a review of cooling flows, astrophysical plasmas, shocks, and interfaces. 

Studies of the high-ionization metal-line absorbers provide insights into hot diffuse gas that has been processed through stars in galaxies. In the ultraviolet and optical bands, these absorbers have been studied primarily using five-times ionized oxygen (OVI), six-times ionized nitrogen (NV), and seven-times ionized neon (NeVIII).  Both OVI and NeVIII arise within the spectral range of the Lya forest and are thus mostly visible at low redshifts where the Lya forest line density is much smaller. NV is adjacent to the Lya line and in principle can be surveyed over the full range of redshift; however, this ion is found in only a narrow range of astrophysical conditions.  The population statistics measured include the redshift path density, the equivalent width and column density distributions, the cosmic mass densities, and the kinematics (broadening parameters, velocity splitting distributions, and absorber velocity widths). In this chapter, we discuss multiple observational programs and their reported findings for several of these ions.

Empirically demonstrating the association of metal-line absorbers with galaxies has a long, rich history from the earliest theoretical predictions in the mid-1960s to observational confirmation in the 1990s. From that point forward, quasar absorption line studies became a powerful tool for characterizing the gaseous halos of galaxies. Countless works have provided valuable insights into the chemical, ionization, and kinematic conditions of what is now called the circumgalactic medium. A new concept called the baryon cycle was birthed in which the balance of accretion modes, stellar feedback, gas recycling, and outflow dynamics of galactic gas was found to be closely linked to how baryons respond to dark matter halos of a given mass.  Modern theory known as halo abundance matching has helped us empirically connect the average stellar mass to dark matter halos of a given mass.  Powerful hydrodynamics simulations tell a story in which the average baryon cycle processes in a galaxy is closely linked to dark matter halo mass. In this chapter, we discuss how synthesizing both the observational data and theoretical insights has yielded a simple composite model of the baryon cycle.

Over the last quarter century, studies of the circumgalactic medium (CGM) have evolved from small, isolated cottage-industry efforts to a few dozen factory-scale assembly-line collaborations. The advent and continued development of large galaxy surveys, the refinement of photometric redshifts, and the honing of color-selection of quasars have all combined to yield million+ object searchable catalogs for building large samples of galaxy-quasar pairs on the sky. Though the largest body of work has focused on low- and intermediate-redshifts, where detailed galaxy properties can be measured, wholesale studies of the CGM have now reached redshifts of 4 using Lyman break galaxies (LBGs) and the stacking of the spectra of thousands of Lyman Alpha Emitters. In this chapter, we provide an overview of CGM studies with a focus on sample building and experimental approaches and techniques.  The three main types of survey strategies are discussed.  Concepts such as the characterization of CGM absorption properties as a function of impact parameters, covering fractions, and galaxy-absorber morphokinematic and morphospatial analysis are presented.

Absorption line studies have shown that the circumgalactic medium (CGM) is an extended complex multiphase gas reservoir of galaxies. It is a kinematically diverse region that interfaces the baryon cycle activity within galaxies to the intergalactic environment in which the galaxies are embedded. In this chapter, selected observational programs and their reported results are presented. The focus is on empirical bivariate relations, such as absorption strength and covering fractions versus impact parameter, stellar mass, star formation rate, etc. The CGM is presented as viewed through several commonly targeted ions, in particular HI, MgII, CIV, OVI, and NeVIII.  Though this allows the various ionization stages of CGM gas to be examined in isolation, it glosses over the multiphase nature of the CGM. The practical design of high redshift experiments is such that they are much more statistical in nature than the more granular experiments at low redshift.  Thus high-redshift studies are discussed separately. 

Galaxies do not live alone; they live in groups and clusters; they are surrounded by smaller companions bound in or passing through their dark matter halos. As such, there is some ambiguity when studying the CGM in connection to “isolated” galaxy properties because gas is shared between galaxies and companions. In this chapter, we describe halo occupation distribution (HOD) theory, which characterizes the average distribution of companions associated with a given galaxy. HOD relations guide our quantified definitions of galaxy groups and clusters and provide a formalism within which absorption line studies can be applied to the intragroup medium (IGrM) and the intracluster medium (ICM). The remainder of this chapter covers the characteristic properties of the IGrM and the ICM. The IGrM is primarily discussed in terms of theoretical hydrodynamic simulations of small groups like our own Local Group. Of interest are the dynamic “boundaries” between the individual CGM of the orbiting member galaxies and the common IGrM envelope. Mergers are briefly discussed followed by a detailed characterization of the ICM based on X-ray emission studies.  

In this chapter, selected observational programs of merging galaxies, groups, and clusters are presented, and their reported results summarized. For each, neutral, low-, and intermediate-, and high-ionization gas is examined separately. Various findings appear to indicate that comparing absorption to the “nearest galaxy,” the “most massive galaxy,” or the “central galaxy,” can strongly influence the inferred conclusions from the studies. The results that appear to agree between the various studies, is that, compared to the CGM of member galaxies, the metal-line selected IGrM gas appears to be both relatively optically thin and kinematically quiescent in both its low- (MgII) and high-ionization (OVI) phases. Clusters, on the other hand, surprisingly appear to have neutral gas deep into their cores and, within the virial radius of the cluster, the sizes of the CGM of the individual member galaxies appears to be diminished compared to galaxies residing outside the virial radius.  At the time of this writing, the study of the CGM in the IGrM and ICM environment is a developing area of study. 

In this chapter, the taxonomy of the emission spectra of starbursts, active galactic nuclei (AGN), and quasars are compared. These spectra are discussed in terms of their emission line diagnostics as measured on Baldwin-Phillips-Terlevich (BPT) diagrams. The non-unique typing of AGN/quasars as Markarian galaxies, LINERs, Seyfert galaxies, radio galaxies, blazars, BL Lac objects, and flat-radio spectrum quasars is explained. The taxonomic subclassification of Seyfert galaxies and quasars based on the relative strengths of permitted broad lines and forbidden narrow lines are discussed. The quasar main sequence, which is based on the kinematics of the Hb emission line and the luminosity ratio of the FeII/Hb emission lines, is introduced. Insights into the nature of AGN/quasars can be gleaned from the fact that their luminosities and spectral energy distributions can be highly variable on timescales of hours to decades.  Broad absorption lines (BALs) and narrow absorption lines (NALs) arise in strong outflows. The BALs may provide clues about viewing angles, leading to radio-quiet and radio-loud unified models of AGN and quasars. 

Black holes were hypothesized as far back as the 1770s, but were not theoretically formalized until 1916, nor observationally identified until the 1970s. Since then, they have been recognized as a ubiquitous and important component of galaxy evolution and the baryon cycle. At the heart of AGN/quasars, they generate powerful outflows, which are believed to be radiatively driven. The nature of these outflows depends on the luminosity generated by the black hole accretion disks and the radiative efficiency of the accretion process. The luminosity is characterized by the Eddington ratio, the ratio of the bolometric luminosity to the Eddington luminosity, which is the value at which radiation pressure propels infalling gas outward. Quasars are observed to have a Schechter function distribution of Eddington ratios. Based on arguments of force multipliers, the case for radiative line-driven winds is advanced. A simplified picture in which outflows can be predicted on a plot of Eddington ratio versus black hole mass is discussed, as well as a black hole evolution H-R type of diagram base on ``downsizing.”

In this chapter, we discuss the energetic outflows from quasars, which achieve velocities 10--20% of the speed of light. BALs are quantified using the ``balnicity index,” an imperfect measure of a complex phenomenon that includes variability, saturation, self-blending, and partial covering. BALs have several subclasses, including HiBALs, LoBALs, and mini-BALs. BAL evolution is not well understood and selected competing models are discussed. Associated narrow line absorption (NALs) can also be present with equally high velocities. Four subclasses of NALs are discussed but characterizing NALs is challenging. Variability, partial covering, and line locking can help their identification. Line locking, in particular, is described in detail as it is a key aspect of radiatively line-driven outflows. Efforts and challenges for determining the fraction of quasars with BALs and NALs are described. In this chapter, we also discuss the quasar CGM, including the proximity effect (both line-of-sight and transverse). The technique of quasars probing quasars (QPQs) is described as are the observed properties of the quasar CGM learned from QPQ experiments.