E. Margaret Burbidge

Annual Review of Astronomy and Astrophysics, vol. 5, p.399

view ADS abstract: 1967ARA&A...5..399B 

This is the very first review article covering quasars (QSOs). It describes their discovery and identification. The article also summarizes their observed properties. There is a large focus on the identification and intepretation of emission and absorption lines, the latter of which were barely understood at the time. There is also an extended discussion on plausible theories of the energy generation, structure of the objects, and emission line formation. Most of the focus is on optical spectra, though some discussion of radio properties is included. Burbidge makes an attempt to reference every work that was published up to the time this article went to press, so this is an excellent resource for completeness of very early, if fluid, knowledge. 

Marteen Schmidt

Annual Review of Astronomy and Astrophysics, vol. 7, p.527

view ADS abstract: 1969ARA&A...7..527S

This article is a quik (yet thorough) follow-up to Margaret Burbidge's 1967 ARA&A review. As Schmidt says in his introduction, this manuscript is essential a progress report for the knowledge gained over the years 1968 through 1969. His article is similarly structured like Burbidge's but expands on complementary topics. Schmidt also points the reader  to several other review articles on the quasi-stellar radio sources by (i) Ryle, (ii) Sheuer & Williams, (iii) Kellerman & Pauliny-Toth, and (iv) Cohen, all of them in the same ARA&A volume as this one by Schmidt. 

William A. Stein, Stephan L. O'Dell, & Paul A. Strittmatter

Annual Review of Astronomy and Astrophysics, vol. 14, p.173

view ADS abstract: 1976ARA&A..14..173S

The first progress report on the so-called BL Lac objects was published when a little more than 30 such objects were known. Thier key properties are summarized, as are their sky distribution, visual appearance, spectral-flux distribution, and polarization parameters. The authors propose that BL Lac objects represent a small range of observable properties that are more generally related to the processes that give rise to the spectra of quasars and Seyfert galaxies.

Paul A. Strittmatter & Robert E. Williams

Annual Review of Astronomy and Astrophysics, vol. 14, p.307

view ADS abstract: 1976ARA&A..14..307S

Written seven years after Schmidt's 1969 review and nine years after Burbidge's 1967 review, this article extends what is observationally known about quasars and the emission and absorption lines in their optical spectra. After a thorough tour of the continua and emission lines of optical quasar spectra, the authors focus on absorption lines. They incorrectly adopt the point of view that absorption lines are primarily intrinsic to the quasars and proclaim that "only very few arise from intervening material."  Under this assumption, outflow models based on line distributions are explored. The authors conlcude that a "breakthrough is imminent in the understanding of these enigmatic objects."

E. Maragaret Burbidge

Physica Scripta, vol. 17, p.165

view ADS abstract: 1978PhyS...17..165B

This article is behind a pay wall, so fewer people have read it and it remains sorely under cited. However, the abstract is available on NASA/ADS.  Here is the abstract: A brief review of work on the emission-line spectra of QSOs is given. Data on radio-emitting and radio-quiet objects are discussed, with sections dealing with the lines that are observed, the distributions of redshifts obtained from the emission lines, and a selection of particular problems posed by current observations. These include a discussion of QSOs with atypical spectra, the Lyman/Balmer/Paschen relative line strengths, the abundances of the elements, correlations of observed properties, and gas flow within the emission-line region.

E. Margaret Burbidge

Physica Scripta, vol. 17, p.201

view ADS abstract: 1978PhyS...17..201B

This article is behind a pay wall, so fewer people have read it and it remains sorely under cited  However, the abstract is available on NASA/ADS.  Here is the abstract: A review of the key discoveries concerning absorption lines in QSOs is given. A number of recent studies bearing on the nature and origin of the absorption are discussed. It is concluded that it is most likely that the bulk of the absorption is intrinsic to the QSOs. (Note that this is the second review article in the later half of the 1970s to incorrectly attribute the majority of absorption lines to the quasar source.)

Alec Boksenberg

Physica Scripta, vol. 17, p.205

view ADS abstract: 1978PhyS...17..205B

This article is behind a pay wall, so fewer people have read it and it remains sorely under cited. However, the abstract is available on NASA/ADS.  Here is the abstract: The physical properties of the absorption systems observed in high redshift QSOs are described in some detail. Apart from the very broad absorption lines seen in a few objects, which almost certainly are due to intrinsic mass outflow, the great majority of the narrow absorption lines commonly present in QSO spectra, ranging in relative velocity from a large fraction of [the speed of light] outward to a few thousand [kilometers per second] inward, all of which are characteristically similar and resemble interstellar lines, can be most naturally explained as being produced in cosmologically distributed intervening material consisting of both enriched and primordial gas.

Ray J. Weymann, Robert F. Carswell, & Malcom G. Smith 

Annual Review of Astronomy and Astrophysics, vol. 19, p.41

view ADS abstract: 1981ARA&A..19...41W

For roughly 15 years, this article was the "starter-kit" for all students and researchers interested in learning about quasar absorption lines. It provided the first mature and essentially correct summary of absorption lines at a time when the field was still nascent. Here, we learn about four types of absorber classes (A, B, C, D), including general descriptions of each type and their redshift distributions. These classes eventually evolved into the well-known modern categories: intervening Lyman-alpha forest lines, associated broad absorption lines (BALs), associated narrow absorption lines (NALs), and intervening metal lines. Also described are techniques of analysis.  The majority of the discussions focus on the origin and nature of both associated (BALs included) and intervening Lyman-alpha and metal-line systems. The article concludes looking toward future unknowns (from the perspective of 1980s astronomy).

Donald E. Osterbrock & William G. Matthews

Annual Review of Astronomy and Astrophysics, vol. 24, p.171

view ADS abstract: 1986ARA&A..24..171O

This is a masterclass discussion of both the observations and theory of emission features from quasars and AGN, with focus on Seyfert 1 and Seyfert 2 classes. Osterbrock was a premier observer of AGN who also had full command of line formation physics and Matthews ranked among the most prominent versatile and insightful theorist of the day. After covering the emission line properties of AGN in great detail, the authors embark on a deep-dive review of the current (circa 1985) models of AGN physics. Any modern student or seasoned practitioner of AGN physics would find this review paper foundational to their research journey.

J.-S. Chen 

Astrophysics & Space Sciences, vol. 118, Iss. 1-2, p.473

view ADS abstract: 1986Ap&SS.118..473C

This highly under-appreciated article presents a brief, tight, but comprehensive review of the status of quasar absorption lines in the mid-1980s. It shockingly has zero citations on NASA/ADS (it is not behind a pay wall!), yet it provide a no-nonsense, clean, and cohesive review. It covers the topics of selection effects, which were still being sorted out in the mid-1980s but remain relevant today, Lyman-alpha and intervening metal lines, and redshift path densities. Each section is summarized with bulleted lists of absorber properties and theoretical interpretations that are quickly diigestible and point to the original references for further investigation.  This is the perfect article for anyone who is looking for the "cliff notes"* (with important updates) to the much longer Weymann etal. 1981 review.

* cliff notes = TL;DR

Frederick D. A. Hartwick & David Schade 

Annual Review of Astronomy and Astrophysics, vol. 28, p.437

view ADS abstract: 1990ARA&A..28..437H

The luminosity function of quasars is a key quantity in that these objects strongly effect the evolution of the ionization balance of the universe and trace the duty cycle of supermassive black hole accretion and AGN in the context of the overall evolution of galaxies. This article focuses on what has been measured up to and slightly into the Cosmic Noon era.  Though z>3 quasars were known to these authors, not enough were known to constrain the evolution of space density quasars for z>3. This article provides clear insight into what what known and what the open questions were at the opening of the 1990s. In the three decades since then, quasars beyond z=10 have been found. We now know that the quasar luminosity function grows rapdily starting at about z=5.3 and marks the onset of the HeII Reionization era. This is a fascinating read for quite a journey that followed -- and it all starts with this article.

Fun Fact: F. Hartwick first coined the term Circumgalactic Medium (CGM) in his 2006 paper "Early Cosmic Chemical Evolution: Relating the Origin of a Diffuse Intergalactic Medium and the First Long-Lived Stars" (2006ApJ...640...41H).

Robert Antonucci

Annual Review of Astronomy and Astrophysics, vol. 31, p.473

view ADS abstract: 1993ARA&A..31..473A

With nearly 4000 citations, this the most referenced ARA&A article on the topic of  AGN and quasars. The abstract is deceivingly understated: "Contents: 1. Introduction. 2. Radio quiet AGN. 3. Radio loud objects. 4. Things to come." This is an amazing article. It presents a succinct history while summarizing (in bullet and tabular formats) the wide range of observations across all spectral bands. Antonucci then sorts through both coherent and incoherent information to formulate a unified model of AGN and quasars. Any beginning student or seasoned researcher basing their work on the unified model must familiarize themselves with this foundational work. The figures may appear dated, but the science is not! It should be a fun challenge for any modern researcher to ascertain which of the "things to come" listed in Section 4 remain unknown and in need of continued research three decades later.

Patrick Petitjean

in "Formation and Evolution of Galaxies", eds. O. Le Fevre & S. Charlot (Springer-Verlag), arXiv:astro-ph/9810418

view ADS abstract: 1998astro.ph.10418P

Ecole des Houches is famous for excellent astronomy summer schools. The 1998 Les Houches summer school "Formation and Evolution of Galaxies" included a review lecture on quasar absorption line systems by Patrick Petitiean. This is one of the first succinct reviews when the field had just modernized, including the advent of high-resolution spectra on 10-meter class telescopes and theoretical interpretations using early computer simulations. Following an illuminating introduction, Petitjean covers line formation and analysis, (curve of growth, Voigt profile fitting) , the Lyman-alpha forest (evolution, column density distribution, physical conditions, velocity clustering, simulations), metal-line systems (association with galaxies, impact parameter distribution, kinematics), and damped-Lyman alpha absorbers (DLAs ,metallicity evolution). This under-cited comprehensive summary of the field remains fresh in its delivery of the cutting-edge analysis methods and science of its time, much of which remains totally relevant today.

Michael Rauch

Annual Review of Astronomy and Astrophysics, vol. 36, p.267

view ADS abstract: 1998ARA&A..36..267R

This very long and incredibly thorough review article covers every aspect of the forest including the history of its prediction, discovery, and the array of physical models that have been employed to explain the velocity clustering, column density, ionization, temperatures, and cosmological distribution of intergalactic and galactic HI absorbing structures. Also discussed is the HeII forest. Though this is the first review of the Lyman alpha forest following the advent of HIRES quasar spectra and the higher powered computer simulations, the discussions are essentially correct; they are not dated, This review remains a valuable resource for those who wish to understand the plethora of observed properties, the cosmic evolution, and physical insight significance of the Lyman alpha forest.

Frederick Hamann & Gary Ferland

Annual Review of Astronomy and Astrophysics, vol. 37, p.487

view ADS abstract: 1999ARA&A..37..487H

This is an exceptional review of associated absorption lines in quasars. The authors cover emission line diagnostics, absorption line diagnostics, metallicities, chemical enrichment scenarios, connections to galaxy chemical evolution, and a discussion of astrophysical implications for early (high-redshift) enrichment in quasars (active galaxy centers). Also included is a brief stand-alone summary of quasar abundances. Similar to Rauch's review of the Lyman-alpha forest, this review is written at a time when the field of associated absorption first broke wide open after years of slow but steady progress, and consequently it is an invaluable snapshot of the foundations of this field. It remains relevant and fresh today.

Jane C. Charlton & Christopher W. Churchill

Encyclopedia of Astronomy and Astrophysics, article id. 2366

view ADS abstract: 2000eaa..bookE2366C

This contribution to the Encyclopedia of Astronomy is a brief introduction to intervening quasar absorption line concepts. The authors cover everything from the Lyman-alpha forest to the kinematics and multi-phase ionization of metal-line systems to the insights from early cosmological simulations. Though the history of surveys is dated to the pre-2000s, the basic concepts are 100% relevant for today's students and early-career researchers. The curve of growth of absorption lines is illustrated and described with exceptional clarity, as is the visual and scientific impact of high-resolution spectroscopy for investigating the chemical, kinematic, and ionization conditions of galactic gas.

Jill Bechtold

Chapter 4 in "Galaxies at High Redshift," p 131 (54 pages)

view ADS abstract: 2001astro.ph.12521B

This longer review article is truly a "one-stop-shop" overview of intervening absorption lines. Virtually every sub-topic is described, ranging from the curve of growth to Voigt profie fitting, from the detailed evolution of the Lyman-alpha forest lines to the kinematic modeling of DLAs, from D/H ratio measurements to the HeII reionization era, and from molecular absorption to the galaxy-absorber connection. As such, Bechtold has provided a excellent snap shot of the cutting-edge of the state of the field at the turn of the millenium (after almost 40 years of quasar science). 

Arthur M. Wolfe, Eric Gawiser, & Jason X. Prochaska 

Annual Review of Astronomy and Astrophysics, vol. 43, p.861

view ADS abstract: 2005ARA&A..43..861W

Ever since the seminal 1986 paper by author Art Wolfe, damped Lyman-alpha absorbers (DLAs) have provided a key astronomical tool for measuring cosmic evolution in the mass density of neutral hydrogen, dust content, molecular content, ionized-gas content, gas kinematics, and galaxy identifications across cosmic time. The gas associated with DLAs appears to be "a multiphase ISM in which radiative and hydrodynamic feedback processes are present." The authors also discuss kinematic and ionization models of DLAs with focus on galaxy formation and evolution. This review is the launching pad for any student interested in the power of DLAs as a class of absorber and DLA-focused research.

Xiaohui Fan, Christopher L. Carilli, & Brian Keating

Annual Review of Astronomy and Astrophysics, vol. 44, p.415

view ADS abstract: 2006ARA&A..44..415F

This article is way ahead of its time in many regards, especially in terms of its predictions of future observations. By the mid-2000s, the Gunn-Peterson trough was a common feature seen in z>6 quasar spectra and this indicated a fairly rapid transition from a homogeneous and highly neutral cosmic medium to one with localized ionization bubbles and shells carved out by the formation of the first luminous objects. After a brief introduction describing the epoch of reionization, the authors engage in discussions of ionization theory, sources of reionization, and the growth of "Strömgren spheres" in the context of the IGM. They journey into the power of 21-cm absorption studies against the comic microwave background (CMB) for measuring the epoch of the first stars and black holes. In all cases, they describe the observational challenges. Today, much more is known observationally about the epoch of reionization, but this article outlines the activities that then took place over the next 20 years. It serves as an excellent starting place for anyone wishing to get acquanted with the history and astrophysics of reionization.

Mary E. Putman, Josh, E. G. Peek, M. R. Joung

Annual Review of Astronomy and Astrophysics, vol. 50, p.491

view ADS abstract: 2012ARA&A..50..491P

This article is highly focused on local universe HI emission in the radio. However, it mixes in quasar absorption line work in order to draw broader insights into the gas complexes surrounding our Galaxy, and for that reason I have included it in this list.  All gas phases are considered, from the cold-neutral medium (CNM) to the warm-hot ionized medium (WHIM), as are all gas structures from the disk ISM to high-velocity clouds, to satellite galaxy streams.

Pushpa Khare

Bulletin of the Astronomical Society of India, vol. 41, p.41

view ADS abstract: 2013BASI...41...41K

Though this article is over a decade old, it should be required reading for every incomng student in the field. An explosion in both observational information and theoretical understanding and predictions occured in the years since the more technique-focused reviews by Petitjean (1998), Charlton & Churchil (2001), and Bechtold (2001). The article has a tight introduction including an annotated, beautiful example of a high-resolution quasar spectrum. The article begins with the IGM including a review of models of Ly-alpha clouds, chemical erichment of the IGM, low-redshift IGM morphology, mean opacity at high reddshift and the Gunn-Peterson Effect, the proximity effect, evolution of the baryon mass density in the IGM, and the HeII forest. After a very brief mention of BALs, the author embarks on intervening metal-line absorbers, including MgII-selected, CIV-selected, DLA-selected, and sub-DLA systems. Both molecular absorption and dust content is reviewed, as is the global cosmic metallicity evolution. The article ends with a discussion of time evolution of fundmanetal constants as measured using quasar absorption lines. A great has been learned since this article organized our field into a coherent overview, but none of the findings presented within have been proven incorrect, so it is an an excellent snapshot of the broad astrophysical questions that can be tackled using quasar absorption lines. 

Matthew McQuinn

Annual Review of Astronomy and Astrophysics, vol. 54, p.313

view ADS abstract: 2016ARA&A..54..313M

This review breaks the IGM into three redshift regimes, (1) z=2-5, (2) z>5, and (3) z<2, with emphasis on z~0.  McQuinn descusses our understanding of the three redshift regimes circa 2015. He describes that we had a high-confidence level for z=2-5, but that both the current epoch was less known due to uncertainties in the IGM/CGM interface with galaxies and the reionization epoch (z>5) was still shrouded in considerable mystery. This article not only reports and describes the cutting-edge knowledge and theory at the time of its writing, but also the observations required to advance that knowledge and theory. Since this article appeared, advances have occurred at all three redshift regimes, but in particular z>5 has been invesitgated both observationally and theoretically (using simulations). This paper is foundational for those wishing to become experts of the IGM and its evolution.

Jason Tumlinson, Molly S. Peeples, Jessica K. Werk

Annual Review of Astronomy and Astrophysics, vol. 55, p.389

view ADS abstract: 2017ARA&A..55..389T

Before the Circumgalactic Medium (a term coined by F. Hartwick in 2006), there simply was the so-called "galaxy halo". It took roughly three-decades since the discovery of quasars to definitively establish that galaxies in fact were surrounded by extended gaseous halos. Many prominent astronomers argued that the halo was an inconsequential part of galaxies and their evolution-- they were very wrong. This paper summarizes the 30 years of hard labor that gave birth to the concept of the CGM and revealed that it is a dynamic, multi-ionization phase, chemically enriched component of  galaxies and contains a significant fraction of the baryonic mass in galaxies. This article summarizes what is observationally known about the CGM and describes the theoretical challenges in interpreting the absorption line data to date, including inflows, outflows, and gas recycling through stars. A great deal of the paper focus on the metal content of the CGM. The authors conclude with "open problems, future prospects, and final thoughts." This paper is a great jumping-off point for learning about the CGM.

Max Pettini

Lectures 11, 12, & 13 of the Course "Introduction to Cosmology" taught at University of Cambridge, UK. 

view the PDF: Lecture11.pdf

view the PDF: Lecture12.pdf

view the PDF: Lecture13.pdf

Pettini is a master teacher who has distilled down an otherwise somewhat challenging set of concepts into a simple and highly accessible form. These three tight expositions on Lyman-alpha forest absorption and its interpretation as the neutral component of the intergalactic medium are the perfect primer for students interested in learning about these topics.

Céline Péroux & Christopher Howk

Annual Review of Astronomy and Astrophysics, vol. 58, p.363

view ADS abstract: 2020ARA&A..58..363P

Over the last two decades, the study of the CGM via quasar absorption lines coupled with both the observations of galaxy emission lines and galaxy evolution theory as studied using cosmological hydrodynamic simulations led to the realization that galaxy evolution is strongly governed by the so-called baryon cycle. If this cycle were perfectly known, then the time dependency of the total mass of baryonic gas and the mass of metals would be fully specified as baryons and metals transition from the IGM to the CGM, from the CGM to the ISM, from the ISM to stars, and from stars back into the ISM, the CGM, and the IGM, and all permutations thereof.  This article is a bold attempt to organize all that is known about the cosmic global evolution of baryons, metals, and dust within the context of galaxies and the total cosmos. As the main measurement tool is quasar absorption lines, the authors have provided key introductory background and explanations of the derived quantities obtained from the analysis of quasar absorption lines. This highly accessible review is both thorough comprehensive.

Megan Donahue & Mark Voit

Physics Reports, vol. 973, p.1

view ADS abstract: 2022PhR...973....1D

This is a monster review paper, almost a miniture book. It is complete with a primer on galaxy evolution and the baryon cycle. Overall, the authors focus on the strong connections between the central supermassive black holes and the extended "atmospheres" around galaxies (including the Milky Way), galaxy groups, and clusters, and finally the massive central galaxies in clusters. They touch on galactic environments for every interest. They then embark on detailed discussions of cosmological accretion, radiative cooling, and coronal heating. They have a full exploration of what they call "Circumgalactic Weather." They summarize by considering how balance is achieved in these processes, including the physical limits on the precipitation of gas onto galaxies and conclude with a discussion on the broader implications for galaxy formation and evolution. Be prepared to buckle in if you want to absorb the whole of this material.

Claude-André Faucher-Giguère & Peng S. Oh

Annual Review of Astronomy and Astrophysics, vol. 61, p.131

view ADS abstract: 2023ARA&A..61..131F

This review on the CGM appears only six years following the 2017 Tumlinson etal ARA&A review paper. This is a testimony to how rapidly studies of the CGM have advanced and how important the CGM is in the minds of astronomers and astrophysicists. Per the title, the emphasis of this article is a deeper dive into the physics and processes occurring in the CGM and their broader role in galaxy evolution. Following an excellent introduction, especially focused on the simulations, the authors embark on census of the main physical processes: accretion and outflowing winds. The role of angular momentum and the influence of satellite galaxies are also described. The article is highly focused on the small scale hydrodynamic physics of gas cloud formation and destruction in the context of dark matter halos. The authors then focus on efforts to resolve these small scale processes in hydrodynamic cosmological simulations. Future prospects are then considered.  This article is ideal for the early-career theoretical astrophycists among us.

Todd A. Thompson & Timothy M. Heckman

Annual Review of Astronomy and Astrophysics, vol. 62, p.529

view ADS abstract: 2024ARA&A..62..529T

This review is a deep dive into galactic winds, which comprise one of the most important components of the baryon cycle and have one of the biggest influences on the physical state of the CGM. This review is effectively two articles in one. First, the authors cover the theoretical aspects of winds, how they are driven, and how they evolve around galaxies of difference masses and star formation rates. Then, the authors embark on a cencus of observations across multiple wave bands. Of course, down-the-barrel UV absorption line observations against the luminous nuclear regions of the galaxies are a major observational tool for understanding winds, and these are featured prominantly in this article.

Nicolas Tejos

to appear in Encyclopedia of Astronomy and Astrophysics

view ADS abstract: 2025arXiv250412539T

Do not let the understated title under sell this review paper. This is a comprehensive up-to-date review of the evolution of cosmic gas that covers the physics of the IGM, ranging from the basic informative equations to the results of the latest cosmological simulations. All five major phases of the gaseous universe are discussed (HM, WHIM, WCGM, DIGM, Halo Gas, and Stars) as well is their cosmic evolution. Most informative are the phase diagrams for the major structuralc components in the Universe, i.e., knots, filaments, sheets, and voids. The baryon budget is covered, as is the evolution of the HI ionization rate and the thermal state. The latter half of this extensive article focuses on quasar absorption lines and their contribution to the science, including curve of growth methods for measuring column densities and thermal broadening, pixel statistics, and the Gunn-Peterson trough. Finally, survey statistics are described, including the HI column density distribution function, The article concludes with a tour of various obseervational techniqies that advance our understanding of the IGM.