COSMOLOGICAL ABSORPTION LINE SPECTROSCOPY

Transcription

1 COSMOLOGICAL ABSORPTION LINE SPECTROSCOPY Introduction to the Principles and Analysis of Quasar Absorption Lines Christopher W. Churchill New Mexico State University, Las Cruces, New Mexico, United States

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3 Contents Preface page i 1 Introduction Discovery of High Redshift Probes Development of the Modern World View The Forest Damped Lyα Absorption Hydrogen Ionization Breaks Probing Galaxy Halos At the Close of Decade Two The 1980s The 1990s In Space On the Ground The Absorber-Galaxy Connection Into the New Millennium 34 2 Cosmological Paradigm Geometry and the Metric Time and Distance Hubble s Law The Robertson-Walker Metric Relativistic Spacetime Dynamics Parameterized Cosmology Dynamics Friedmann-Einstein Cosmologies The Robertson-Walker Metric The Development Angle ΛCDM, H 0, and the Ωs 56

4 4 Contents Evolution of the Ωs The 737 Cosmology 62 3 Applied Cosmology Redshift Observer Time Time Dilation Expansion Dynamics Distances, Separations, and Angles in Cosmology Co-Moving and Proper Radial Distances Radial Separations Transverse Separations Angular Diameter Distance Angular Separations Proper Separations in Lenses Source Beam Cross Section Luminosity Distance Surface Brightness Absorption Distance Cosmological Volume Peculiar and Recessional Velocities Peculiar Velocities Recessional Velocities 99 4 Hydrogenic Atoms The Bohr Atom Energy Structure and Transitions The Schrödinger Atom The Schrödinger Equation Properties of the Wave Function The Schrödinger Wave Function Energy Structure and Transitions The Dirac Atom Spin Spin-Orbit Coupling Energy Structure and Transitions Radiative Corrections Isotope Shifts Continuum States Energy Structure and Transitions 128

5 Contents 5 5 Transitions of Hydrogenic Atoms Transition Probabilities The Dipole Approximation Einstein Coefficients Schrödinger Oscillator Strengths Averaged Oscillator Strengths The Radial Overlap Integrals Lyman and Balmer Series Oscillator Strengths Fine-Structure Oscillator Strengths Oscillator Strengths for Multiplets The Spectrum Bound-Free Oscillator Strengths Natural Broadening Emission Power and Absorption Cross Section Bound-Bound Emission Bound-Bound Absorption Bound-Free Absorption Multi-Electron Atoms and Transitions The Many-Electron Problem The Hartree-Fock Method Coupling Schemes The Russell-Saunders Vector Model Russell-Saunders Term and State Symbols Energy Structure Central Field Approximation L S Coupling Spin-Orbit Coupling: Fine Structure j j and Intermediate Coupling Schemes Bound-Bound Transitions Oscillator Strengths and Line Strengths Emission Power and Absorption Cross Section Selection Rules Isotope Shifts Atomic Properties and Spectra Ground-State Russell-Saunders Symbols The Periodic Table Iso-Sequence Ions Binding Energy and Ionization Potential Grotrian Diagrams and Spectra 200

6 6 Contents Group IA: Alkali Metals Group IIA: Alkali-Earth Metals (and Helium) Group IIIA: Boron Group Iron Family Ions Common Absorption Lines Radiative Transfer The Radiation Field The Geometry The Specific Intensity The mean Intensity The Photon Field The Flux Vector Defining the Observed Flux Measuring the Observed Flux Radiative Transfer The Extinction Coefficient and Optical Depth The Emission Coefficient The Transfer Equation and Source Function Solution in One Dimension Absorption Lines and Optical Depth Transforming Energy Densities Pure Absorption The Astronomical Absorption Spectrum Cosmological Intervening Absorption Associated Absorption and Partial Covering Column Density Spectrographs and Spectra The Spectrograph The Seeing Disk Slits Diffraction Gratings The Grating Equation Free Spectral Range The Interference Function The Blaze Function The Intensity Function Blazing a Grating Low-Order Spectrographs Echelle Spectrographs 279

7 Contents Resolving Power Diffraction Limited Resolution Seeing Limited Resolution Instrumental Spread Function Convolution of the ISF Pixelization Pixel Plate Scale Dispersion per Pixel Spectroscopic Data Atmospheric Attenuation and Throughput The Recorded Spectrum: Flux to Counts Uncertainties in Recorded Counts Flux Calibration Absorption Lines and Ionization Breaks The Challenge of Spectral Analysis Absorption Lines The Total Absorption Cross Section The Thermal Broadening Function The Voigt Profile Equivalent Width and the Curve of Growth Curve of Growth Methods Additional Physics and the Curve of Growth Apparent Optical Depth Method Partial Covering from Doublets Observed Redshifted Equivalent Widths Ionization Breaks Spectral Analysis Continuum Fitting Combining spectra Objectively Measuring Absorption Lines Aperture Method An Optimized Method The Line List Absorption Line Systems Systemic Redshift Equivalent Widths and Mean Wavelengths Kinematics Rest-Frame Velocities Velocity Moments 365

8 8 Contents Fractional Velocity Width Two-Point Velocity Correlation Function The Apparent Optical Depth Method Apparent Optical Depth Profiles Apparent Optical Depth Column Densities Deblending Absorption Features Gaussian Decomposition Voigt Profile Decomposition Gas Physics and Processes The Radiation Field In the Intergalactic Medium In the Circumgalactic Medium The Particle Field Particle and Mass Density Conservation Abundances and Mass Fractions Charge Conservation and Ionization Balance Thermodynamic Equilibrium Thermalized Velocity Distributions Kinetic Energy Pressure The Equation of State Detailed Balancing Excitation Excitation Rates and Thermodynamic Equilibrium Excited States and Electron Density Ionization Photo and Auger Ionization Direct Collisional Ionization Excitation Auto-Ionization Radiative Recombination Dielectronic Recombination Charge Exchange Astrophysical Rates and Rate Coefficients The Cooling Function Photoionization Heating Collisional Ionization Cooling Recombination Cooling Collisional Excitation Cooling Continuum Cooling 440

9 Contents Compton Heating and Cooling The Net Cooling Curve Ionization Modeling Rate Equations Hydrogen Helium Metals Equilibrium Solution Ionization Equilibrium Ionization in Thermodynamic Equilibrium Applying Ionization Models to Data Cloudy The Ionization Parameter Ionization Structure Ionization Grids Properties of Ionization Grids Constraining Metallicity and Abundances Estimating Cloud Sizes and Masses Caveats and Complications of Ionization Modeling Intervening Quasar Absorption Lines Surveys Redshift Path Redshift Path Density Equivalent Width and Column Density Distributions Mean Gas Density Two-Point Velocity Clustering Multiple/Closely Paired Sightline The Circumgalactic Medium Galaxy-Absorber Pairs Halo Covering Fractions Cross Sectional Sizes of Halos Halo Gas Phases Halo Gas Mass The Intergalactic Medium Gunn-Peterson Effect The Lyα Forest The He ii forest The Epoch of Reionization 501

10 10 Contents References 508 Index 531

11 Preface This is the preface. Parents. Teach your children well.

12 1 Introduction The discovery of quasars... made a new science out of astronomy. This opening proclamation 1 by D. Weedman (1986) in the preface of his book, Quasar Astronomy, should be considered a resolved fact. After nearly six decades of quasar 2 research, it is almost impossible to conceive of the state of extragalactic science prior to quasars. In the early 1960s, when quasars entered our collective conscience, spontaneous matter creation in a steady state expanding universe (Holye, 1948) was a leading theory of cosmology; in truth, there wasn t really a consensus on the cosmological model of the universe. At the time, most astronomers accepted the view that the universe was expanding (Hubble, 1929), and this was the only observationally supported keystone upon which a paradigm of cosmology could be built. Galaxy evolution as a scientific discipline had not yet been born; galaxies were still referred to as the nebulae. Theories of galaxy evolution and formation (e.g., Eggen et al., 1962) were in their infancy. Dark matter, though observationally motivated by Zwicky (1937), wasn t taken seriously. The cosmic background radiation had not yet been discovered. The Lyα forest simply didn t exist, theoretically or observationally. The concept of a cosmic web, megaparsecs of intercrossing filaments of neutral hydrogen networking galaxy clusters together, was unconceived and 35 years away. Observations that would suggest the existence of dark energy 1 The statement has been slightly modified from D. Weedman s 1986 original,whichreads The discovery of quasars nearly a quarter of a century ago made a new science out of astronomy. 2 The term quasar was coined by Hong-Yee Chiu in 1964, originally to denote a quasi-stellar radio source, or an object that was discovered by its radio brightness and then observed to be unresolved in optical images, i.e., stellar-like in appearance. Papers into the mid-1970s referred to them as QSSs. As some objects were found to exhibit theunique,non-stellar optical spectral properties of quasars but not be detected in the radio, a sub-classification of quasi-stellar objects (QSOs) was adopted. Throughout this work, we adopt the term quasar to interchangeably denote a quasi-stellar radio source (QSS), or a quasi-stellar object (QSO).

13 2 Introduction (e.g., Riess et al., 1998; Schmidt et al., 1998; Perlmutter et al., 1999b) were 40 years away. Technologically, optical spectra were commonly measured using photographic plates; photomultipliers and channel scanners were being developed and refined during the 1960s and 1970s. The very first astronomical application of a charged couple device (CCD) had to wait until 1975; CCDs were not common until the mid-1980s. 1.1 Discovery of High Redshift Probes In 1960, it all began when a 16th magnitude star was routinely identified as the only optical object within the error radius of the published position of the radio source 3c 48. In October of that year, A. Sandage acquired the optical spectrum only to find a mysterious pattern of broad emission lines. A second radio source, 3c 273, was soon observed in December 1962, and some months later M. Schmidt (1963) announced that the mysterious emission lines were Mgii λ2800 and the Balmer series at the large redshift z = λ obs λ r λ r =0.158, (1.1) where λ r is the rest-frame wavelength and λ obs is the observed wavelength. Following Schmidt s announcement that 3c 273 was a highly redshifted object, 3c 48 was determined to have a redshift z = In Figures 1.1(a,b), we reproduce the photographic plates of the spectra of 3c 48 and 3c 273 presented by Greenstein & Schmidt (1964). Greenstein & Schmidt state that the spectrum of 3c 48 was sufficiently abnormal to show this object was not an ordinary star or an extragalactic nebula 3 of moderate redshift. By 1966, information derived from the spectra of only nine quasars had been published (Greenstein & Schmidt, 1964; Schmidt & Matthews, 1964; Schmidt, 1965). The first publication that proposed using absorption to probe the properties of a cosmic medium along the line of sight to quasars was by Shklovsky (1964), who suggested that the resonant Mgii λλ2796, 2803 doublet should be observed blueward of the Mgii emission line. No absorption was seen in the very early available data. Assuming a reasonable ionization fraction and metallicity, he estimated the first upper limit on the cosmic neutral hydrogen density at n H atoms cm 3. In 1964, the redshift of the quasar 3c 9 was measured by M. Schmidt (see Schmidt, 1965) to be z em =2.01. This was twice that of the next highest redshift source and provided the first observable Lyα emission line in a 3 An extragalactic nebula refers to a galaxy.

14 (a) 1.1 Discovery of High Redshift Probes 3 (b) Figure 1.1 Reproduction of the photographic plate spectra of 3c 273 and 3c 48. (a) The spectrum of 3c 273, the first quasar for which the emission lines were recognized to be redshifted (z =0.158, Schmidt, 1963). The spectrum was first obtained in December [Fig. 2 of Greenstein & Schmidt (1964)] (b) The spectrum of 3c 48, the first radio source for which an optical object was identified. The spectrum was first obtained in October Following M. Schmidt s announcement that 3c 273 was a redshifted object, 3c 48 was determined to have a redshift z = [Fig. 3 of Greenstein & Schmidt (1964)] quasar spectrum (shifted to 3659 Å). In January, Scheuer (1965) published a Nature paper entitled A sensitive test for the presence of atomic hydrogen in intergalactic space. Not having access to the spectrum, Sheuer suggested that if the spectrum lacked a general flux decrement blueward of the Lyα emission line that (1) the density of hydrogen in the intergalactic medium is tiny, (2) intergalactic hydrogen is primarily ionized, or (3) the redshift of the quasar is not cosmological (so that line of sight to 3c 9 does not probe intergalactic medium). Interestingly, Sheuer did not predict discrete absorption lines from neutral clouds embedded in an ionized medium. Similar to the work of Scheuer, Gunn & Peterson (1965) reasoned that if neutral hydrogen was present in the intergalactic medium along the line of sight, then Lyα absorption would cause a flux decrement blueward of

15 4 Introduction the Lyα emission line; having access to the Schmidt s spectrum, they measured the flux blueward of Lyα emission, and submitted a paper in late May 1965 (Scheuer was not referenced). Gunn & Peterson assumed that intergalactic space was a smooth uniform gaseous medium. They concluded that intergalactic hydrogen at z = 2 is highly ionized via photoionization by a ubiquitous radiation field from one of four possible sources: normal galaxies, radiogalaxies, QSSs, and/or the intergalactic medium itself. One challenge was that the Lyα emission line of 3c 9 falls near the ultraviolet atmospheric cutoff so only a small redshift range could be analyzed. Thus the original Gunn-Peterson measurement was performed in the physical vicinity of the quasar. Though no one was discussing proximity effects, it would later be acknowledged that the quasar itself is effective at ionizing its surrounding intergalactic medium. That same May of 1965, the first absorption line recognized in a quasar spectrum was published by Sandage (1965). He wrote, On both plates the blueward [emission] line is easy to see and is bisected by a sharp absorption feature, flanked by the broad emission. We interprete this to mean selfabsorption. The best candidate for the broad emission and sharp absorption lines were Civ, whichyieldedaredshiftz em = Kinman (1966) presented the second reported discovery of absorption lines in his spectrum of PHL 938 (now known to have z em =1.954). The discovery ignited searches for more examples of absorption and a debate on whether the absorption lines are due to self-absorption as quickly suggest by Sandage (1965), or due to gas within intervening galaxy clusters as suggested by Bahcall & Salpeter (1965). Some suggested that both types of absorption may be occurring. By 1970, (Bahcall, 1970, 1971) had introduced the terms Type I to designated self-absorption and Type II to designate intervening absorption. The first stages of this debate were engaged in the literature when Burbidge et al. (1966) reported that the quasi-stellar radio source 3c 191 had numerous absorption lines in its optical spectrum. The photographic spectrum by Burbidge et al. is illustrated in Figure 1.2. A higher quality spectrum of 3c 191 was later obtained by Bahcall et al. (1967). This spectrum, as presented in Figure 1.3, represented the most detailed view of a complex quasar spectrum to that time. Seven spectra with absorption lines were reported in the literature between by the end of 1966 (Sandage, 1965; Burbidge et al., 1966; Bahcall et al., 1966; Ford & Rubin, 1966; Kinman, 1966; Lynds et al., 1966; Schmidt, 1966; Stockton & Lynds, 1966). Of these, 3c 191 would continue to remain in the spotlight and be the subject of many observational campaigns.

16 1.1 Discovery of High Redshift Probes 5 Ly α (em) C IV (em) Figure 1.2 The photographic spectrum of 3c 191 obtained at the prime focus of the Lick 120-inch telescope in February Multiple absorption lines are identified. Note the strong Civ absorption bisecting the Civ emission (slightly to the blue wing) and the multiple silicon lines associated with the Lyα absorption. [Fig. 2 of Burbidge et al. (1966) with minor adaptations] NS Si III abs Lyα em NS C IV em SKY COUNTS Si IIabs Si IIabs C II abs Ly α abs Si IVem Si IVabs C IV abs Figure 1.3 Microphotometer tracings of the spectrum of 3c 191 and of the spectrum of the adjacent night sky (both spectra have the same vertical scale, but are offset for presentation purposes). The spectral resolution is 3.7 Å. Sky lines are from city lights (mercury). The spectrum was obtained by M. Schmidt in January 1967 using the prime focus spectrograph on the Palomar/Hale 200-inch telescope. [Fig. 1 of Bahcall et al. (1967) with minor adaptations]

17 6 Introduction The notion of multiple absorption redshifts was strongly promoted by Burbidge et al. (1968). The abstract of their paper simply reads, Evidence is presented that quasi-stellar objects having large emission redshifts can have a multitude of absorption-line redshifts. Bahcall & Salpeter (1965) estimated that if neutral hydrogen is concentrated in intervening galaxy clusters, some seven absorption lines per unit interval of redshift should be observed on average (for estimated cluster parameters at that time, which were extremely crude). Bahcall & Salpeter (1966) further extend their work and, for assumed gas phases and metallicities, predicted a typical galaxy cluster would give rise to absorption lines from Nv, Civ, Siiv, Siiii, Siii, Mgii, Mgi, Feii. Interesting, there is no mention of Ovi absorption. Savedoff (1956) appears to have been the first to propose that spectral lines observed in cosmological objects can be used to examine whether fundamental atomic constants change with time. Of particular interest is the fine structure constant, α = e 2 / c. In the closing of their paper on the absorption lines in 3c 191, Bahcall et al. (1967) measure the ratio α(z =1.95)/α(z = 0) = 0.98 ± 0.05 from the Siiv λλ1393, 1402 doublets and the Siii λ1260 and λ1527 transitions. Wagoner (1967) suggested that individual intervening galaxies could be the source of the absorption lines. He wrote, The probability that photons received from a distant object have passed near enough a galaxy to be significantly modified is computed and found to be appreciable for cosmological redshifts z>1. Wagoner was the first to formulate that the probability that a line of sight from a quasar at redshift z em is intercepted by gas associated with a galaxy is P (z) = n(m)σ(m)dm 0 z em f(z)dl(z), (1.2) where M is the magnitude of a galaxy, n(m) is the density of galaxies with magnitude M, σ(m) is the mean cross section of the absorbing gas associated with a galaxy of magnitude M, and where f(z) accounts for the (unknown) redshift evolution of the magnitude distribution, and dl(z) is the proper path length element at redshift z. Wagoner assumed that the gaseous cross sections of galaxies was effectively equal to their luminous cross section. With an eye toward the redshift controversy of quasars, he concluded if these lines are not present at the expected fraction of large redshift sources, then either (a) galaxies of moderate redshift (assumed cosmological) are much different than those nearby, or (b) the sources are not at cosmological distances. Shklovsky (1967) should also be credited with early work along these lines.

18 1.2 Development of the Modern World View 7 Even in view of all this activity, in his Annual Reviews article entitled Intergalactic matter, Gould (1968) discussed the efforts and contributions of quasar research for characterizing the gaseous component of intergalactic matter. He summarizes, In fact, one might summarize all such attempts at detecting intergalactic matter by observations of quasars by simply stating that no positive identifications have been made. Such sentiments would change in the 1970s. Shortly thereafter, in one of the most understated, but correct propositions for the decade, Bahcall & Spitzer (1969) write (as their entire abstract), We propose that most of the absorption lines observed in quasi-stellar sources with multiple redshifts are caused by gas in the extended halos of normal galaxies. Their central assumption is that the cross sections of the gas surrounding the galaxies are much more extended than the optical emission sizes assumed by Wagoner. Bahcall & Spitzer show that the probability of the sightline from a z em quasar will intercept a galaxy halo at redshift z is (q 0 =0.5) ( ) R 2 N 0 (1 + z) 9/2 P (z) =2 100 kpc 0.03 gal Mpc 3 [ (1+zem ) 3/2 1] 1+z (1.3) where the typical halo size is assumed to be R = 100 kpc and the nonevolving average density of galaxies is assumed to be N 0 =0.03 galaxy Mpc Development of the Modern World View The Forest By 1970, the two highest redshift quasars known, PHL 957 (z em =2.681) and 4c (z em =2.877), were famous for having complex absorption line patterns, and it was particularly of note that the number of absorption line blueward of Lyα were several factors of ten times higher than redward of the emission. Lynds (1971) was the first to suggest most absorption lines shortward of Lyα emission are themselves Ly α absorption. The column densities of the putative Lyα absorbers were suggested to be so small that even strong associated absorption from the resonant Civ λλ1548, 1550 would not be detectable. Much later in the decade, Boksenberg (1978) proposed that the Lyα absorbers are primordial, that the gas is pristine and not polluted by metals. This became a (perceived) characteristic for defining what became known as Lyα forest lines. Bergeron & Salpeter (1970) and Arons & Wingert (1972) should be credited with being the first to exploit the known properties of these putative

19 8 Introduction Lyα absorption lines to place constraints on a theory of a patchy intergalactic medium of neutral hydrogen clouds ionized by ultraviolet radiation from quasars. In particular, Arons & Wingert found that a universal medium of T = 10 4 K intergalactic hydrogen clouds ionized by the radiation from quasars is consistent with the Lyα lines. (a) (b) Figure 1.4 (a) A high resolution, 0.42 Å, television camera spectrum of PHL 957 over the wavelength range Å captured with a SEC vidicon in a 6 hour exposure on the Palomar/Hale 200-inch telescope. This is narrow wavelength segment of high resolution and high sensitivity selected based upon analysis of two image tube spectra covering Å at a resolution of 10 Å and at 5 Å resolution. [Fig. 4 of Lowrance et al. (1972)] (b) The extracted and calibrated spectrum from the television vidicon facility. [Fig. 5 of Lowrance et al. (1972)] In a first attempt to measure the distribution of neutral hydrogen column densities and gas temperatures, Morton & Morton (1972) modeled several of the Lyα lines using the Voigt profile. A Voigt profile results from the convolution of the natural atomic width of a transition (a Lorentzian) and the line of sight thermal properties of the gas (a Gaussian). Morton & Morton are the first to present the caveats of their pioneering work:... the present analysis assumes that each absorption line can be represented by a Doppler-damping profile formed in a homogeneous medium with broadening by only a Maxwellian velocity dispersion and the natural damping of the

20 1.2 Development of the Modern World View 9 transitions. The method of Voigt profile fitting to quasar absorption lines was soon thereafter widely adopted. (a) (b) Figure 1.5 (a) The four spectra used by Peterson (1978) for counting Lyα lines: PKS (z em = 3.52), PKS (z em = 2.88), PKS (z em =2.68), and PKS (z em =2.21). [Fig. 4 of Peterson (1978)] (b) The redshift path density, dn/dz, obtained from the four spectra in panel (a). The diamonds provide the breadth of the redshift coverage and estimate error in the number of lines per unit of redshift. The smooth curves are no-evolution expectations (Eq. 1.4) for q 0 = 0 (upper curve) and q 0 =0.5 (lower curve). [Fig. 5 of Peterson (1978)] As data quality and sample sizes increased over the first half of the 1970s, it became apparent that the number of narrow lines blueward of the Lyα emission was greater in the higher redshift quasars. If the Lyα absorbing structures have a characteristic cross section, σ 0, and cosmic number density, n 0, and are cosmologically distributed intervening Type II absorbers, then the number of lines counted in a fixed redshift bin, dn/dz, are expected to increase at higher redshift according to dn dz = c 1+z n 0 σ 0, (1.4) H 0 (1 + 2q 0 z) 1/2 where H 0 is the Hubble constant and q 0 is the deceleration parameter (see Chapter 3 for details). Peterson (1978) was the first to exploit this relationship and, using the four quasars illustrated in Figure 1.5(a), showed that the product n 0 σ 0 decreased rapidly from z =3toz = 2. His preliminary result is illustrated in Figure 1.5(b). At the time even apparently strong signatures, as illustrated in Figure 1.5, were difficult to interprete because of the lack of control in the uniformity of the data and analysis. For such reasons, the very same year as Peterson s work, Perry et al. (1978) stressed the importance of selecting samples for which all absorption included in an analysis is above a chosen minimum

21 10 Introduction rest-frame equivalent width. A few years later, Sargent et al. (1980), in a seminal work that confirmed Lyα forest lines are cosmologically distributed, applied such uniformity to the analysis Damped Lyα Absorption In the z em =2.69 object PHL 957, there is particularly strong absorption at λ4022 between the Lyα and Lyβ emission lines. Assuming it is Lyα, the redshift is z = 2.31, a significantly lower redshift than the quasar. Targeting this line, Beaver et al. (1972) obtained a spectrum of PHL 957 using the UCSD Digicon mounted on the Lick 3-meter telescope; the 8 Å resolution spectrum is illustrated in Figure 1.6. As part of their analysis of the strong absorption, Beaver et al. superimposed a damped Lyα profile over the data. However, they interpreted the broad system, and the multiple narrow lines observed by Lowrance et al. (1972), to arise in gas associated with or ejected by the quasar (i.e., Type I absorption). As such, the opportunity to suggest the intervening nature of the what appears to be the first detailed study of a damped Lyα absorber (DLA) was missed. In his examination of PHL 957, Wingert (1975) singled out the damped Lyα absorption first studied by Beaver et al. (1972) as arising in a possible metal-enriched galaxy halo, stating, The system at z equals is found to resemble interstellar material in ionization and abundance ratios and constitutes strong evidence for absorption originating in a galaxy along the line of sight to the quasar. Other early examples of damped Lyα absorption systems and their analysis can be found in Carswell et al. (1975a) (Q ) and Jauncy et al. (1978) (PKS ) Hydrogen Ionization Breaks In the rest frame of the absorbing gas, the ionization of neutral hydrogen with large optical depths will remove photons with wavelengths less than 912 Å (the Lyman continuum) from the light beam. The cross section for ionization scales as σ λ (λ/912) 3 such that an Hi column density of cm 2 yields a residual flux of e 1 =0.368 at the ionization edge of 912 Å. For larger column densities the transmission will vanish, so that no flux is detectable below the ionization edge. For redshifted absorption, the observed flux decrement will be observed at λ obs = 912(1 + z). In the optical (above the ultraviolet atmosphere cutoff), the Lyman ionization edge is observable for z>2.4. Thus, in the 1970s, only the highest redshift quasars could be examined for this feature.

22 1.2 Development of the Modern World View 11 (a) (b) (c) Figure 1.6 (a) A photographic plate spectrum of the quasar PHL 957, highlighting strong Lyα absorption at λ4022 (z = ). There are two strong sky emission lines to the red of the absorption. [Fig. 5 of Beaver et al. (1972)] (b) The post sky-subtracted Digicon measurements (dots) for the wavelength range marked in panel (a). An optimal interpolation spectrum is superimposed and a damped Lyα profile with N(Hi) = cm 2 is centered on the 30 Å wide Lyα absorption line. [Fig. 3 of Beaver et al. (1972)] (c) A microphotometer scan of the photographic plate [note the strong night sky (NS) lines]. [Fig. 4 of Beaver et al. (1972)] Strong flux discontinuities in quasar spectra at the Lyman edge of a redshifted Lyα absorber are now referred to as Lyman limit breaks and the absorption systems are referred to as Lyman limit systems (LLS). By 1976, the presence or non-presence of Lyman discontinuities had been investigated in four quasars (Carswell et al., 1975b). By 1979, fifteen z>3 quasars were known. Osmer (1979) examined fourteen of these fifteen high redshift quasars at a low resolution of 40 Å, perfectly suitable for detecting Lyman discontinuities. Two of his spectra, Q and Q , are presented in Figure 1.7 as logarithm of rest-frame frequency versus logarithm

23 12 Introduction Figure 1.7 Spectra of Q and Q shown in rest-frame frequency obtained with the SIT vidicon system on CTIO at a resolution of 40 Å. Both spectra exhibit strong flux drop outs of the Lyman continuum, due to the ionization edge of the Lyman series. Data obtained July October 1977 [Taken from Figs. 1(c) and 1(d) of Osmer (1979)] of monochromatic flux, f ν erg s 1 cm 2 Hz 1. In a prophetic conclusion, Osmer states that the majority of Lyα absorption lines in quasar spectra have column density too low to cause Lyman discontinuities, and they are too numerous to arise from intervening galaxies. He also states that the Lyα lines likely arise in a tenuous intergalactic medium, and then notes that the number of Lyman limit systems are consistent with galaxies if the cross sections are kpc. Examination of evolution of Lyman discontinuity number counts to lower redshifts would have to wait until the advent of space-based telescopes, since the Lyman ionization edge is observable only below the ultraviolet atmosphere cutoff for z< Probing Galaxy Halos As pointed out by Wagoner (1967), a statistical connection between absorbers and galaxies would help settle the debate of whether quasar redshifts were cosmological by establishing the existence of Type II absorption. The supposition that the bright galaxies have extended halos (Bahcall & Spitzer, 1969) would eventually prove to be the most widely accepted interpretation and provide very powerful support for the Type II absorber hypothesis. Radio 21 cm surveys were being conducted to search for absorbers with high neutral hydrogen columns densities; a most successful technique was to target redshifts of large Lyα absorption (Wolfe et al., 1976, 1978; Wolfe & Davis, 1979). During this time, Haschick & Burke (1975) were the first to report 21 cm absorption associated with a galaxy, NGC 3067, in the spectrum of 3c 232 (also known as 4c 32.22). The velocity difference between

24 1.2 Development of the Modern World View 13 absorption and the galaxy is 76 km s 1. If the absorption arises in the disk, it lies at a radius of 60 h 1 50 kpc. Prior to space-based telescopes, Ca ii λλ3934, 3969 and Nai λλ5891, 5897 doublets were the only optical interstellar absorption lines that could be measured in galaxies having low enough redshifts that the galaxies could be unambiguously identified. As such, the Caii doublet was the first known optical quasar absorption lines firmly established to arise in the halo of a galaxy, and this galaxy was also NGC 3067 toward the quasar 3c 232 (Boksenberg & Sargent, 1978). A Digital Sky Survey image of the 3c 232/NGC 3067 quasar-galaxy pair (centered on the quasar) and the associated Caii doublet absorption at z = in the spectrum obtained by Boksenberg & Sargent are shown in Figures 1.8(a) and 1.8(b), respectively. (a) 5 x 5 (b) 3c 232 NGC 3067 Figure 1.8 The first optical atomic absorption lines associated with a galactic halo. (a) Digital Sky Survey image centered on the z em = quasar 2c 232. The z = galaxy NGC 3067 is 1.9 away projected on the sky, at an impact parameter of 16.5 h 1 50 kpc to the 2c 232 line of sight. [Image obtained from NED] (b) IPCS spectra from the Palomar/Hale 200-inch telescope of 3c 232 showing the Caii λλ3934, 3969 doublet at the redshift of NGC [Fig. 2 of Boksenberg & Sargent (1978)] Surveys for absorbing galaxy candidates ensued; in one of the very first, Weymann et al. (1978) imaged six quasar fields and, for follow-up examination, identified the mag objects in the range kpc radial separation (impact parameter) from the quasars. They conclude that, if galaxies are associated with Mg ii absorbers, the typical separation of the galaxy from the absorbing cloud will be several tens of kiloparsecs. During the late 1970s, there were some additional reported cases of absorption associated with galaxies, but, even by decades end, the cases were deemed to be mostly ambiguous (see Weymann et al., 1981). Burbidge et al. (1977) built upon the original idea as introduced Wagoner (1967) to reexamine the possibility that the absorption spectra of QSOs

25 14 Introduction are due to disks, coronae, or halos of intervening galaxies. They develop the fundamental formalism (see ) for inverting the redshift path density of absorbers, dn/dz, to constrain the physical sizes of gaseous halos. They conclude that the number of absorbers can only be consistent with absorption from galaxies if the typical halo sizes are R 100 kpc. One way to interprete this result is that Burbidge et al. (1977) provided a statistical confirmation of the proposal by Bahcall & Spitzer (1969); however, they rejected the possibility of large halos and instead conclude that the bulk of the absorption is likely to be intrinsic to the QSOs. (a) (b) (c) CIV Mg II Figure 1.9 (a) A segment of the IPCS spectrum of Q , highlighting a resolved Civ system. Two components are identified at a resolution of 0.71 Å, one at z = and [From Fig. 1 of Boksenberg & Sargent (1975), the lower axis is channel number] (b) The Mgii doublet at z =0.424 in the spectrum of PKS , resolved with the IPCS at 0.41 Å. [Lower panel from Fig. 1 of Boksenberg et al. (1979)] (c, upper) A four-component Voigt profile model of the Mgii λ2803 absorption. (c, lower) The model superimposed on the data after convolution with the instrumental spread function. [Fig. 2(b) of Boksenberg et al. (1979)] Since high-resolution studies are technology driven, those with access to large telescopes and sensitive detectors had first rights. One of the most productive detectors during the 1970s and early 1980s was University College London s Image Photon Counting System (IPCS, Boksenberg & Burgess, 1972), a versatile instrument that was mounted on various telescopes throughout the globe. The ICPS was sensitive enough that high dispersions on the order of 0.5 Å were achievable on the Palomar/Hale 200-inch telescope. Using the IPCS, Boksenberg & Sargent (1975) resolved several absorbers at a resolution of 0.71 Å. An example profile from their spectrum of Q is presented in Figure 1.9(a). A two component Civ λλ1548, 1550 doublet,

26 1.2 Development of the Modern World View 15 at z = and z = (a 175 km s 1 velocity splitting), were resolved in this single system. As illustrated in Figures 1.9(b) and 1.9(c), Boksenberg et al. (1979) provide a forerunner of the type and detailed level of analysis that was to become common practice in the late 1990s and beyond. Using the ICPS, they resolved the z =0.424 Mgii doublet in the spectrum of PKS at a resolution of 0.41 Å (panel b). In an exploration of the physical properties of the individual components, Boksenberg et al. modeled the kinematics, column densities, and Doppler b parameters of each cloud using Voigt profiles (upper panel c). To fit the data, the model profiles are convolved with the instrumental spread function (lower panel c). In conclusion, Boksenberg et al. state that the characteristics of the z = complex are consistent with the interpretation that the absorption arises in gas associated with an intervening galaxy in the line of sight to PKS They then note that PKS is one of the six quasars imaged by Weymann et al. (1978) for which they identified a candidate galaxy for the Mgii absorber a year earlier. (a) (b) (c) Figure 1.10 (a) The geometric configuration for the kinematic model for a spherically symmetric halo embedding a galactic disk as developed by Weisheit (1978). A rotation model is defined by the function V rot ( ω, z), where ω is the projected radial distance out on the disk and z is height above the disk for location r on the line of sight (LOS) vector; the dot product with the line of sight yields the line of sight velocity, V (r). [Fig. 1 of Weisheit (1978)] (b) The Lyα, Civ λ1548, and Ovi doublet profiles for one selected set of halo model parameters (see Weisheit s paper for details). Dashed lines are symmetric profiles for comparative illustration. [Fig. 3(a) of Weisheit (1978)] (c) The absorption profiles of the same four transitions, but for the radiative acceleration clouds model, which essentially is two clouds with different column densities offset in velocity. [Fig. 3(b) of Weisheit (1978)] To investigate observed kinematics, and motivated the Type I/II debate, Weisheit (1978) developed galaxy halo models. His first model was a spher-

27 16 Introduction ically symmetric rotating halo enveloping a galactic disk, the configuration for which is illustrated in Figure 1.10(a). The optical depth is obtained from the line of sight velocity probability distribution function, P (v), based on a kinematic function, V (r), which is the dot product of the line of sight with the velocity field, a thermal dispersion function T (r), and a density distribution, n(r). In Figure 1.10(b,c), Lyα, Civ, and Ovi profiles are illustrated for the adopted halo model parameters. The importance of Weisheit s kinematic model is that it was firmly based upon the notion of absorption arising in disks and extended halos of galaxies. In the following decades, variations of models based essentially on these methods would be developed (e.g., Lanzetta & Bowen, 1992; Charlton & Churchill, 1998; Steidel et al., 2002; Chelouche & Bowen, 2010). Starting in the late 1990s, the sophistication of kinematic models would reach new heights using computer models of the hydrodynamics of forming and evolving galaxies in the context of structure growth in a cosmological setting. 1.3 At the Close of Decade Two During the 1970s, the problem of the absorption lines had moved to center ring, almost completely overshadowing the question of the nature of the quasars. The questions surrounding quasar absorption lines became the siren for the seventies; their scientific lure was so compelling, that Weymann et al. (1981) were moved to speculate that more observing time on large telescopes has been spent on spectroscopy of QSO absorption lines than on any other program. The two Annual Reviews articles that summarize quasar spectroscopy research for the decade were focused almost entirely on absorption line studies (Strittmatter & Williams, 1976; Weymann et al., 1981), a drastic shift from the emphasis of the two Annual Reviews articles summarizing the 1960s (i.e., Burbidge, 1967; Schmidt, 1969). The 1970s saw the first bonafide quasar catalogs, starting with the compilation of 202 quasar with redshifts and full bibliography up to June 1971 (de Veny et al., 1971). A half-decade later, some 380 quasars were cataloged by Barbieri et al. (1975) summarizing the literature from December And soon thereafter, Burbidge et al. (1977) published a catalog of 633 quasars with established redshifts taken from the literature up to August A bit over a year later, Perry et al. (1978) reviewed the status of absorption lines from their perspective; they also presented an updated catalog of over 800 quasars with measured emission redshifts. Some 120 of these quasars had reported absorption systems, though not all were confirmed. In 1978, the International Ultraviolet Explorer (IUE) was launched and

28 1.4 The 1980s 17 flew until The IUE allowed quasar spectroscopy below the atmospheric cut off ( Å) for the first time. By decade s end, the modern science of quasar absorption lines was in full swing. In the ten year span (i) photographic plates scanned by photometric sensors were replaced by image-tube scanners and photon counters that had higher spectral resolution, linear response, and quantifiable noise characteristics, (ii) the cosmological origin of quasar redshifts was virtually universally accepted; any debate over the interpretation of quasar redshifts was kept alive only by an unaccepting minority, (iii) the Lyα lines were being investigated as probes of a postulated intergalactic medium, (iv) metal-line absorption was being resolved into multiple velocity components, and (v) the galaxy halo hypothesis was on the brink of confirmation. 1.4 The 1980s As the 1980s approached, methods were developed for spectroscopic analysis that included iterative techniques for determining the continuum, line detection significance, and wavelength dependent error propagation for equivalent widths and line counts (Young et al., 1979). These methods were highly instrumental in maturing the study of quasar absorption lines. The time had come for statistically sizable surveys with uniform characteristics, i.e., all spectra obtained with a single instrument and setting, a singularly defined sensitivity criteria, and objective line finding and identification algorithms. The astronomical application of CCDs and improvements in computing power and storage would breathe new life into the science by the latter half of the 1980s. Heading into the decade, researchers were trying to make sense of the plethora and variety of absorption lines. As a further step to refining the categories of the absorption features, Weymann et al. (1981) propose to replace the Type I and II absorption classification of Bahcall (1970, 1971) with four types: A, B, C, and D. Type A absorbers are broad, and reside blueward of the corresponding emission line with relative velocities up to z abs z em /(1 + z em ) = 0.1, yielding v =0.1c, i.e., 30, 000 km s 1. Inclusion as Type A requires non- Lyα absorption broader than 2000 km s 1 that extends at least 5000 km s 1 from the emission line peak. Type B absorbers are sharp/narrow lines residing within ±3000 km s 1 of the emission line peak. Type C absorbers are the most complex. They are sharp/narrow lines with v > 0.01c and must include metal-line absorption. Three Type C sub-classes were characterized as (i) low ionization systems with strong Lyα most often accompanied by

29 18 Introduction Mgii, Feii, Siii, and Cii, (ii) moderate ionization systems that show many of the transitions of low ionization systems with the addition of Siiv and Civ absorption, and (iii) high ionization systems with weaker Lyα, prominent Nv and Ovi and weak or non-existent low-ionized transitions. Finally, Type D absorbers are sharp/narrow lines with v >0.01, blueward of Lyα emission that comprises a very large number of Lyα absorption lines, known as the Lyα forest. Type C Type B Type C Type A Type C Type C Figure 1.11 The spectrum of Q showing the Civ emission and examples of Type A, B, and C absorption lines as proposed by Weymann et al. (1981). Type D systems would not appear in this region of the spectrum. [Adapted from Fig. 1 of Weymann et al. (1981)] In Figure 1.11, we illustrate absorber Types A, B, and C as originally presented by Weymann et al. (1981). Note that Type D absorption would reside blueward of the Lyα emission line and is not shown. Type A absorbers were interpreted to arise in large scale gas flows and/or the bulk motion of multiple blended clouds ejected from quasar. Later, the term broad absorption line (BAL) was adopted, and quasars exhibiting Type A absorption were called BALs or BAL QSOs (see Turnshek, 1986, and references therein). Type B absorbers were interpreted to arise in clouds either associated with the quasar or material in or surrounding galaxies bound to the cluster hosting the quasar (e.g., Foltz et al., 1986). These systems became known as narrow absorption lines systems (NALs).

30 1.4 The 1980s 19 In Weymann et al. (1981), the authors elaborate that Type C and Dhave been the focus of controversy and lament an adequately large sample of high quality data does not yet exist to reliably define the relative occurence of these systems as a function of redshift and column density. Such samples are the legacy of the 1980s. By decades end, further refinement of the classes of absorption systems were established: [1] Lyα forest absorbers, [2] Lyman limit systems (LLS), [3] damped Lyα absorbers (DLAs), Ovi, Civ, and Mgii metal-line systems, [4] broad absorption lines systems (BALs) and, [5] narrow absorption line systems (NALs). A major development was the Palomar-Green Bright Quasar Survey (PG, BQS: 114 quasars) undertaken by Schmidt & Green (1983). During this time, the Palomar/Hale 200-inch telescope was employed by a group of (mostly) Caltech astronomers led by W.L..W. Sargent to observe more than a hundred quasars with uniform spectra for the purpose of settling important issues related to absorption lines, such as the Lyα forest (e.g. Sargent et al., 1980; Young et al., 1982a), the Lyman limit systems (e.g. Sargent et al., 1989; Steidel, 1990b), the Mgii λλ2796, 2803 doublet selected absorbers (e.g. Sargent et al., 1988b; Steidel & Sargent, 1992), and the Civ λλ1548, 1550 doublet selected absorbers (e.g. Young et al., 1982b; Sargent et al., 1988a; Steidel, 1990a). The Caltech efforts were very exciting and lucrative programs. A dozen questions regarding the fundamental nature of intervening absorption line systems that had been posed since the mid-1960s were not only mostly resolved, but, in addition, the gross statistical properties and cosmic evolution of the absorber populations were quantitatively characterized. From these works, the cosmic evolution, redshift clustering, equivalent width distribution, and physical sizes of the absorbing gas structures were quantified. A few selected results highlighting the scientific progress of the 1980s are shown in Figure Of note is the evolution observed in Civ absorbers. From the no-evolution expectation given by Eq. 1.4, we see that we can write dn/dz (1 + z) γ,whereγ =0.5 for q 0 =0.5, and γ = 1 for q 0 = 0. Whenever γ is inconsistent with that range, evolution is inferred. For Civ, Sargent et al. (1988a) obtained γ = 0.9, indicating that the product of the cosmic number density and gas cross section increase with cosmic time (Figure 1.12(c)). As for the chemical and ionization conditions of Civ absorbers, Bergeron & Stasińska (1986) showed that the gas is (1) consistent withbeing photoionized by the cosmic ultraviolet background radiation, (2) has temperatures and densities similar to Galactic gas, and (3) is generallty more highly ionized than Galactic gas, but that the ionization level decreases with

31 20 Introduction (a) Ly α dn/dz (b) Ly α n(w) (c) CIV dn/dz (d) CIV n(w) Figure 1.12 (a) The Lyα forest redshift path density for 1.7 z 3.1 with maximum-likelihood evolution fit N (z) =N 0 (1 + z) γ,withγ =0.5 ± 0.5. [Fig. 1 of Sargent et al. (1980)] (b) The distribution of Lyα rest-frame equivalent widths, fit by an exponential, n(w )=(N /W ) exp{ W/W }, where the maximum-likelihood fit yields W =0.36, given by line A. If line B is the true underlying distribution, when blending is taken into account, line C is observed. [Fig. 2 of Sargent et al. (1980)] (c) The Civ absorber redshift path density for 1.3 z 3.3 with maximumlikelihood evolution fit N (z) =N 0 (1 + z) γ,withγ = 0.9. The absorbers evolve, and it was surmised that chemical enrichment with cosmic time was a candidate process. [Fig. 5 of Sargent et al. (1988a)] (d) The distribution of Civ λ1549 rest-frame equivalent widths, fit by an exponential, n(w ) = (N /W ) exp{ W/W }, where the dotted line is the maximum-likelihood fit for W =0.41. [Fig. 9 of Sargent et al. (1988a)] increasing Hi column density. In subsequent years, work based on ionization modeling by using grids spanning several to-be-constrained parameters, such as density, temperature, and the shape and intensity of the ionizing radiation, became central to developing insights into the nature of intervening absorbers. For Mgii absorbers, evidence for the galaxy halo hypothesis was mounting in that Bergeron (1986) took on the campaign to identify z 0.5 galaxies associated with aborption lines by obtaining spectroscopic confirmation that the galaxy resided at the same redshift as the absorption lines in the quasar spectra. Though high-resolution spectrosopy would hit full stride in

32 1.5 The 1990s 21 the mid-1990s, Tytler et al. (1987) and Petitjean & Bergeron (1990) took the first steps to examine the gas kinematics and velocity clustering of the Mgii absorbers. In addition to being the first to discover that the fraction of stronger absorbers increases toward higher redshifts and that there is a lack of evolution in the redshift path density for systems having equivalent widths greater 0.3 Å, Petitjean & Bergeron examined the two-point velocity clustering functions, and conluded that the two scales explained the velocity clustering of Mgii absorption systems; (1) galaxy halos, and (2) galaxy groups/clusters. 1.5 The 1990s With the foundation of the 1980s firmly in place, the 1990s commenced the golden age of quasar absorption lines spectroscopy. First, the 1990s witnessed the first wholesale spectroscopy in the ultraviolet due to the introduction of space-based observatories. Second, in the optical, the spectral resolution was increased by a factor of ten with the advent of the 10-meter class telescope. By the end of the decade, what was considered a highly specialized, if not somewhat fringe, technique driven branch of astronomy, matured into multiple science driven sub-disciplines. As illustrated in Figure 1.13, the ultraviolet regime below 3200 Å opened studies of the Lyα forest for z 1.6, Mgii for z 0.2, Civ for z 1.1, Ovi for z 2.1, and LLS for z 2.5. The increase in resolution for groundbased astronomy now meant that the internal kinematics of the absorbing gas could be resolved better than v = 10 km s 1 in the Lyα forest for z>1.6, Mgii for 0.2 z 2.6, Civ for 1.1z 5.5, Ovi for 2.1 z 8.6, LLS for z 2.5. However, the highest redshift absorption would require the discovery of ever higher redshift quasars; the highest redshift quasar in 1990 was PC at z =4.73 (Schneider et al., 1989). By the end of the decade, only seven z>4 quasars had been confirmed, with the highest at z =5.82 (SDSS J Fan et al., 2000) In Space The International Ultraviolet Explorer (IUE), launched in 1987, was decommissioned in It was equipped with the Short and the Long Wavelength Spectrographs, which covered the wavelength ranges of Å and Å, respectively. Each spectrograph had resolutions of 0.2 Å and 6 Å, respectively. The resolution and sensitivity was such that, for quasars, primarily only DLAs and LLS absorption systems could be studied (e.g,

33 22 Introduction 6 Ultraviolet Optical Infrared 5 HI 912 OVI 1031 HI 1215 Redshift 4 3 CIV MgII Observed Wavelength, Angstroms Figure 1.13 The observed wavelengths of the major absorber classes, Hi, Ovi. Civ, and Mgii as a function of absorber redshift. Three regions of visibility, infrared, optical, and ultraviolet, are shown. The ultraviolet window opened with IUE ( ) and HST (1990 ). The infrared window, which would allow Civ surveys for z>5 and Mgii surveys for z>2.6 would crack open with the Subaru/ICRS instrument (Kobayashi et al., 2000) and then open wide with the Magellan/FIRE instrument (Simcoe et al., 2013). Lanzetta et al., 1995b). In addition, the absorbing gas in the Galactic halo was studied using high-ionization metal lines for the first time (e.g., Sembach & Savage, 1992). The Hopkins Ultraviolet Telescope (HUT ) was flown twice on the Space Shuttle and operated from the payload bay. It was active in December 1990 (aboard STS-35) and in March 1995 (aboard STS-67). The spectrograph covered the wavelength range Å with a spectral resolution of 3 Å. In April 1990, the Hubble Space Telescope (HST ) was launched on STS-91. Among the original suite of instruments were the Faint Object Spectrograph (FOS, Harms & Fitch, 1991) and the Goddard High Resolution Spectrograph (GHRS, Ebbets & Brandt, 1983), both of which operated until the second servicing mission in February Depending on the mode, HST/GHRS delivered a resolutions of 0.6, 0.06, or Å over the range 1050 to 3200 Å. HST/FOS delivered a spectral resolution of Å in the ultraviolet (1150 Å to 3000 Å), the instrument unfortunately lost sensitivity below

34 1.5 The 1990s 23 f λ erg cm -2 s -1 Ang Lyman Limit Lyε Lyδ FOS/G190H OVI 1031,1037 CIII 977 SiIII 1206 Ly γ Lyβ NII 989 Lyα SiII 1260 f λ erg cm -2 s -1 Ang CII 1334 Galactic FeII 2587,2600 SiIV 1393,1402 Galactic MgII 2796,2803 z= metal line system CIV 1548,1550 FOS/G270H Wavelength, Ang Figure 1.14 An example of an ultraviolet HST/FOS quasar spectrum emphasizing the rich mixed-ionization Lyman-limit absorption system at z = HST/FOS opened a new window on the universe and delivered ultraviolet quasar spectra that were comparable in resolution and signal-tonoise ratio to the optical spectra obtained with 4-meter class ground-based telescopes. Roughly 100 quasar spectra of this quality were obtained by the FOS instrument Å. A typical HST/FOS spectrum of a particularly rich absosption-line system is presented in Figure The HST/FOS facility provided sensitive data on the Lyα lines for redshifts 0 z 1.6. The QSO Absorption Line Key Project (Bahcall et al., 1993) was established to chart the low redshift universe in absorption, including the Lyα forest (e.g., Jannuzi et al., 1998; Weymann et al., 1998), Lyman-limit systems (e.g., Stengler-Larrea et al., 1995), metal-line absorption (e.g., Bergeron et al., 1994; Jannuzi et al., 1998) and Galactic absorption (e.g., Savage et al., 1993, 2000). Highlights of new findings from the QSO Absorption Line Key Project on the Lyα forest and LLS evolution from HST/FOS observations are presented in Figure Most exciting (see Figure 1.15(b)) was the clear transition at z 1.7 in the redshift path density, dn/dz, of the Lyα forest (recall the work of Peterson, 1978). Using hydrodynamic cosmological simulations, Davé et al. (1999) concluded that, at z>1.7, universal expansion drives the

35 24 Introduction (a) LLS dn/dz (b) Ly α dn/dz (c) Ly α n(w) Figure 1.15 (a) The redshift path density of LLSs for 0.5 z 4.1 with maximum-likelihood evolution fit N (z) =N 0 (1 + z) γ,withγ =0.95. Different binning suggests a broken power-law per the findings of Lanzetta et al. (1995b) using IUE. [Fig. 5 of Stengler-Larrea et al. (1995)] (b) The evolution in Lyα forest redshift path density for 0 z 4 indicating a broken power-law of the form N (z) =N 0 (1 + z) γ,withγ =0.16 for 0 z 1.5 and γ =1.85 for 1.5 z 4. [Fig. 6 of Weymann et al. (1998)] (c) The differential distribution of Lyα rest-frame equivalent widths, fit by an exponential with W =0.27 [Fig. 4 of Weymann et al. (1998)] rapid evolution, but starting at that redshift, a diminishing ultraviolet radiation due to the rapid drop in the quasar cosmic number density, counters expansion and slows the evolution. The Space Telescope Imaging Spectrograph (STIS, Woodgate et al., 1998) was installed on HST in February 1997 during the second servicing mission. It experienced electronic failure in August 2004 and was shutdown until its repair in May 2009 during the fourth servicing mission. As of 2018, the instrument remains active. HST/STIS has both optical and ultraviolet capabilities. The ultraviolet has two settings; one covering the near-ultraviolet (NUV) between 1600 and 3100 Å and the other covering the far-ultraviolet (FUV) between 1150 and 1700 Å. Though several resolution modes are available, the instrument provided the first high-resolution cosmological spectra

36 1.5 The 1990s 25 in the ultraviolet. For the limiting flux levels of quasars, the highest resolution achievable in practice was roughly 0.13 Å at 2000 Å. Over the next 15 years, HST/STIS was responsible for breakthrough observations of the Heii Gunn-Peterson trough (e.g., Heap et al., 2000), the low-redshift Lyα forest at high resolution (e.g., Davé & Tripp, 2001), multiple close lines of sight to measure the transverse sizes and kinematic correlation of Lyα clouds (e.g., Aracil et al., 2002), the physical properties of the warm-hot intergalactic medium at low redshift (e.g., Richter et al., 2004), the physical properties of Ovi selected absorbers (Thom & Chen, 2008), the high-ionization phases of mixed-ionization metal-line systems (e.g., Charlton et al., 2003), and the high-resolution metal-lines in Galactic high velocity clouds, (e.g., Herenz et al., 2013) On the Ground As moderate resolution optical quasar spectra continually being collected for various science programs, Hewitt & Burbidge (1993) published what was affectionately referred to as The Phone Book. Their catalog of all known quasi-stellar objects (QSOs) having measured emission redshifts (as of December 1992) contained 7315 objects 4 and provided extensive information such as alias names, redshifts, coordinates, magnitudes, colors, absorption redshifts, variability, polarization, and X-ray, radio, and infrared data. It seemed as if every serious absorption line spectroscopist owned a personal hard copy. The Caltech programs of the 1980s transitioned into the 1990s with a comprehensive compilation of Mgii selected absorbers by Steidel & Sargent (1992). Selected results from their work and summarization of the evolution of the varois classes of absorbers is presented in Figure The differential evolution in the Mgii absorber population reported by Petitjean & Bergeron (1990) was quantified. Qualitatively, as one examines subsets of stronger and stronger Mgii absorbers, the redshift evolution becomes more and more pronounced in that the strongest systems evolve away the most dramatically, whereas the weaker systems exhibit no evolution (see Figure 1.16(a,b)). One interpretation was that the halos are continually being replenished and/or recycled by processes that give rise to the thinner gas structures, but the processes that gave rise to the largest gas structures were functioning only at the higher redshifts. High resolution spectra were sought in order to distill out whether the gas kinematics evolved or the column densities evolved (or 4 For comparison, the SDSS-DR12 Quasar Catalog (Pâris et al.,2017)contains297,301quasars.

37 26 Introduction (a) MgII dn/dz (b) Evolution Parameter, γ (c) MgII n(w) (d) log(dn/dz) LLS CIV (e) R* MgII DLA Holmberg R* Figure 1.16 (a) The Mgii absorber redshift path density for systems with W > 0.3 Å over 0.2 z 2.2. The maximum-likelihood evolution fit to N (z) =N 0 (1 + z) γ yields γ =0.8, consistent with no-evolution in the absorber properties. [Fig. 11 of Steidel & Sargent (1992)] (b) Evolution is apparent as the minimum equivalent width of the sample is increased; γ increases as W min increases, indicating that the strongest systems evolve away the most rapidly with cosmic time. [Fig. 10 of Steidel & Sargent (1992)] (c) The distribution of rest-frame equivalent widths, n(w ), which is equally well fit by an exponential (short-dashed curve) or a power law (long-dashed curve). [Fig. 6 of Steidel & Sargent (1992)] (d) The logarithm of absorber redshift path densities as of 1993 for Civ, Mgii, LLSs, and DLAs. The curves are the predicted values if the absorption is due to gaseous halos with sizes equal to the Holmberg radius of spiral galaxies for q 0 = 0 (dotted) and q 0 =0.5 (dashed) cosmology. [Fig. 4 of Lanzetta (1993)] (e) Using the formalism of Burbidge et al. (1977), the redshift path densities can be inverted to estimate the size of the absorbing gas halo of L galaxies for each class of absorber. This result provided a statistical argument that LLS and Mgii absorption probe the same gas structures. [Fig. 1 of Steidel (1993b)]. both). As shown in Figure 1.16(d,e), as early as 1993, the overall results of systematic absorption line studies provided a statistically based insight into the extent of gaseous halos surrounding galaxies, showing that higher ionization gas would likely be seen to greater projected distances from galaxies. In March 1993, the High-Resolution Echelle Spectrometer (HIRES, Vogt et al., 1994) was successfully commissioned on the 10-meter Keck I tele-

38 1.5 The 1990s UM 18 z= MgII 2796,2803 z= CIV 1548,1550 z= Counts Wavelength, Ang Figure 1.17 The spectrum of the z em =3.024 quasar UM 18 obtained with the Double Spectrograph on the Palomar/Hale 200-inch. This spectrum is typical of 4-meter class telescopes of the era. Note that the two highlighted absorption doublets, though identifiable, appear quite indistinguishable from one another. (insets) The Keck/HIRES spectrum of this quasar resolves the individual members of the doublet absorption and reveals very different insights into the kinematic structures, number of components, and line broadening. [The Double Spectrograph spectrum is courtesy of C. Steidel] scope. HIRES is a grating cross-dispersed, echelle spectrograph capable of operating between 3000 and 10,000 Å and providing resolutions between roughly R = λ/ λ = c/ v of 25,000 to 85,000. For reference, the slit yields R = 45, 000, which resolves absorption lines with velocity widths of v =6.6 kms 1, whereas the typical moderate resolution up to that time corresponds to v 60 km s 1. In Figure 1.17, we qualitatively illustrate the improved insights into the absorption lines provided by the HIRES instrument on Keck. Examples of highly resolved intervening Civ and Mgii absorption are shown in the inset panels. HIRES started a revolution and an explosion of quasar absorption line research that can no longer be fully tracked by any single individual. Scientific quandaries that had been ripening for decades from hard-won cumulative efforts on 4-meter class telescopes were now plucked like ripe hanging fruit in a few nights of telescope time. Young doctoral students and postdocs with access to Keck via the grace of their advisors scraped the cream the column density distributions and the kinematics and chemical and ionization conditions of the Lyα forest (e.g., Hu et al., 1995; Lu et al., 1996), Civ systems (e.g., Rauch et al., 1996), Mgii systems (e.g., Churchill et al., 1996), and DLAs (e.g., Prochaska & Wolfe, 1996). Cosmologically important science, such as the primordial cosmic D/H ratio at high redshifts (e.g., Tytler et al., 1996), yielded to the new technology.

39 28 Introduction f λ x 10 15, erg s 1 cm 2 Ang 1 I λ / I 0 λ (a) (b) Ly β / O VI z=1.961 (panel b) z=2.155 (panel c) z=2.290 z=2.188 Ly α z= (c) 1 I λ / I 0 λ Wavelength, Ang Figure 1.18 (a) Spectrum of a redshift z em =2.423 quasar covering Å in the quasar rest frame. (b) The observed spectral corresponding to the redshift range z for intervening Lyα absorption. (c) An expanded view over the redshift range z The spectral resolution is Å. [spectrum courtesy of M. Murphy] Now, the Lyα forest could be studied at a resolution of 6kms 1 with an order of magnitude increase in sensitivity. The Lyα forest, as observed in the era of 10-meter class telescopes, is illustrated in Figure As highlighted in Figure 1.19(a,b), by the mid-1990s, Voigt profile analysis (recall the pioneering work of Morton & Morton, 1972) of HIRES spectra yielded the column density and Doppler b parameter distributions of the high-redshift Lyα forest. In Figure 1.19(c,d), we present the two-point velocity clustering function of Civ and the Doppler b parameter distribution from Voigt profile fitting of HIRES spectra (Rauch et al., 1996, also see Petitjean & Bergeron (1990)). The velocity clustering of Voigt profile components is consistent with a simple model (dotted curve) of absorbers with small internal velocity dispersion, coupled with a nonisotropic expansion of randomly oriented sheets of clouds. Prior to such a detailed view, the physical interpretation of the Lyα forest was based on a simplified suite of models of spheres, slabs, and dark matter mini-halos (see Meylan, 1995, and references therein). With the

40 1.5 The 1990s 29 (a) Ly α n(n) (b) Ly α n(b) (c) CIV two point velocity correlation (d) CIV n(b) Figure 1.19 Results of Voigt profile fitting to the Lyα forest at z 4 from HIRES spectra. (a) The column density distribution follows a power law with f(n) N β, where β 1.5, with some variation depending on the assumptions of line blending. [Fig. 6 of Lu et al. (1996)] (b) The line broadening Doppler b parameter, b = 2kT/m, where T is the gas temperature and m is the mass of the hydrogen atom. The distribution mode is b 23 km s 1, corresponding to T 30, 000 K. [Figs. 7 of Lu et al. (1996)] (c,d) Results of Voigt profile fitting to Civ absorbers at z 2 from HIRES spectra. (c) The two-point velocity clustering function of Civ absorbers [Fig. 4 of Rauch et al. (1996)] (d) The Doppler b parameter with thermal mean 7.2kms 1 giving a gas temperature of T 38, 000 K. [Fig. 2 of Rauch et al. (1996)] rapid growth in computing power, hydrodynamic cosmological simulations with photoionization from a ubiquitous ultraviolet background were beginning to reveal that baryonic gas gravitationally followed the large scale structure of dark matter density fluctuations, forming a cosmic web of neutral gas stretched into filaments connecting the individual galaxy dark matter halos during the process of hierarchical structure formation (e.g., Cen et al., 1994). This was a revolutionary view of large scale structure, which still had not yet been mapped observationally. To ascertain of the Lyα forest arose from this cosmic web, mock quasar absorption line surveys were performed through simulations and synthetic HIRES spectra were created, analyzed,

41 30 Introduction and quantitatively compared to the data (e.g., Zhang et al., 1995; Davé et al., 1997). With the success of these comparisons, and a growing acceptance that hierarchical structure formation models correctly predicted the collapse of baryonic gas under the influence of dark matter gravity into flattened or filamentary structures, and that it is these structures that are observed in absorption against background quasars, Rauch (1998) summarized the new paradigm of the Lyα forest. In May 1998, the 8.2-meter Very Large Telescope (VLT) saw first light, and by 1999 the Ultraviolet and Visual Spectrograph (UVES, Dekker et al., 2000) began producing spectra of the same caliber as the Keck/HIRES facility. UVES is a dual-beam, grating cross-dispersed echelle spectrograph. For a 1 slit, the resolution is R = 40, 000. By narrowing the slit, it is possible to achieve R 80, , 000. By January 1999, the 8-meter Subaru Telescope saw first light, following which the High Dispersion Spectrograph (HDS, Noguchi et al., 1998) demonstrated its capabilities matched those of UVES and HIRES The Absorber-Galaxy Connection It is during the 1990s that the absorber-galaxy connection was firmly established. Association with Mgii absorption was first targeted because the redshifted doublet appears in the optical for low and intermediate redshifts (i.e., 0.1 z 1), where galaxies are bright enough to image and their emission lines also appear in the optical. Though a few cases had been found in the late 1980s (see Lanzetta & Bowen, 1990, and references therein), it was the seminal paper by Bergeron & Boissé (1991) that provided a dozen clear cases in which a bright galaxy (M r 21) resides at the same redshift as Mgii absorption within a few to ten galaxy Holmberg radii of the quasar line of sight. A flurry of activity followed (e.g., Bergeron et al., 1992; Le Brun et al., 1993; Steidel et al., 1994; Lanzetta et al., 1995a; Bowen et al., 1995; Guillemin & Bergeron, 1997) supporting the bright galaxy-absorber connection due to extended gaseous halos. Some works found exceptions, including the notion that faint-blue galaxies contribute to the absorber population (e.g., Steidel & Dickinson, 1992; Steidel et al., 1993). Of particular note is the work of Yanny & York (1992) who propose that the absorption is not due to extended galaxy halos but rather to groups of 30 to 100 star-forming irregulars or blue compact galaxies. The latter findings would be more consistent with the proposal by York et al. (1986) that many metal-line absorbers arise from gas-rich dwarfs, which naturally cluster around brighter galaxies.

42 1.5 The 1990s 31 By 1993 it was common to read articles entitled The history of halo gas in normal galaxies (Steidel, 1993a) and QSO absorption lines: Implications for galaxy formation and evolution (Lanzetta, 1993). The first comprehensive characterization of the properties of the absorbing galaxy population was reported by Steidel et al. (1994) and Steidel (1995), who examined the luminosity and color distributions, and constrained both the covering fraction and the luminosity dependence of the Holmberg scaling of the size of the gaseous halos (see Figure 1.22(a,b)). However, the important affects of selection bias and incompleteness still required consideration (e.g., Charlton & Churchill, 1996). Figure 1.20 The WFPC-2 image of the field centered on the z em =0.927 quasar 3c 336. A survey of the galaxies in this rich field yielded five identified with Mgii absorption. [Fig. 1 of Steidel et al. (1997)] As illustrated in Figure 1.20, by the latter 1990s, deep imaging with HST combined with ground-based spectroscopic surveys to obtain galaxy redshifts allowed some of the first in-depth analysis of the morphologies of the galaxies (Le Brun et al., 1997; Steidel et al., 1997). As highlighted in Figure 1.21, the HIRES spectrograph allowed the kinematics of the Mgii λ2796 absorption associated with galaxies to be resolved at v 6kms 1.One of the first questions was whether the kinematics behaved in a systematic