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Quasistellar Objects: InterveningAbsorption Lines1

Jane C. Charlton and Christopher W. Churchill

The Pennsylvania State University, University Park,PA 16802

Abstract

We briefly review, at a level appropriate for grad-

uate students and non-specialists, the field of quasar

absorption lines (QALs). Emphasis is on the interven-

ing absorbers. We present the anatomy of a quasar

spectrum due to various classes of intervening absorp-

tion systems, and a brief historical review of each ab-

sorber class (Lyman-alpha forest and Lyman limit

systems, and metal-line and damped Lyman-alpha

absorbers). We also provide several heuristic exam-

ples on how the physical properties of both the inter-

galactic medium and the gaseous environments asso-

ciated with earlier epoch galaxies can be inferred from

QALs. The evolution of these environments from z=4

are discussed.

1. Introduction

Every parcel of gas along the line of sight toa distant quasar will selectively absorb certainwavelengths of continuum light of the quasar dueto the presence of the various chemical elementsin the gas. Through the analysis of these quasarabsorption lines we can study the spatial distri-butions, motions, chemical enrichment, and ion-ization histories of gaseous structures from red-shift five until the present. This includes the gasin galaxies of all morphological types as well asthe diffuse gas in the intergalactic medium.

1.1. Basics of Quasar Spectra

Figure 1 illustrates many of the common fea-tures of a quasar spectrum. The relatively flat

1Written for the Encyclopedia of Astronomy and Astro-physics (to be published in 2000 by MacMillan and theInstitute of Physics Publishing)

quasar continuum and broad emission featuresare produced by the quasar itself (near the blackhole and its accretion disk). In some cases, gasnear the quasar central engine also produces “in-trinsic” absorption lines, most notably Lyα, andrelatively high ionization metal transitions suchas C iv, Nv, and Ovi. These intrinsic absorp-tion lines can be broad [thousands or even tensof thousands of km s−1 in which case the quasaris called a broad absorption line (BAL) QSO], ornarrow (tens to hundreds of km s−1). However,the vast majority of absorption lines in a typi-cal quasar spectrum are “intervening”, producedby gas unrelated to the quasar that is locatedalong the line of sight between the quasar andthe Earth.

A structure along the line of sight to thequasar can be described by its neutral Hydrogencolumn density, N(H i), the number of atoms percm2. N(H i) is given by the product of the den-sity of the material and the pathlength along theline of sight through the gas. Each structure willproduce an absorption line in the quasar spec-trum at a wavelength of λobs = λrest(1 + zabs),where zabs is the redshift of the absorbing gasand λrest = 1215.67 Å is the rest wavelengthof the Lyα transition. Since zabs < zQSO,the redshift of the quasar, these Lyα absorp-tion lines form a “forest” at wavelengths blue-ward of the Lyα emission. The region redwardof the Lyα emission will be populated only byabsorption through other chemical transitionswith longer λrest. Historically, absorption sys-tems with N(H i) < 1017.2 cm−2 have been calledLyα forest lines, those with 1017.2 < N(H i) <1020.3 cm−2 are Lyman limit systems, and thosewith N(H i) > 1020.3 cm−2 are damped Lyα sys-tems. The number of systems per unit redshiftincreases dramatically with decreasing columndensity, as illustrated in the schematic diagramin Figure 2. Lyman limit systems are definedby a sharp break in the spectrum due to absorp-tion of photons capable of ionizing H i, i.e. thosewith energies greater than 13.6 eV. The optical

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Fig. 1.— Typical spectrum of a quasar, showing the quasar continuum and emission lines, and the absorption linesproduced by galaxies and intergalactic material that lie between the quasar and the observer. This spectrum of the z = 1.34quasar PKS0454 + 039 was obtained with the Faint Object Spectrograph on the Hubble Space Telescope. The emissionlines at ∼ 2400 Å and ∼ 2850 Å are Lyβ and Lyα. The Lyα forest, absorption produced by various intergalactic clouds,is apparent at wavelengths blueward of the Lyα emission line. The two strongest absorbers, due to galaxies, are a dampedLyα absorber at z = 0.86 and a Lyman limit system at z = 1.15. The former produces a Lyman limit break at ∼ 1700 Åand the latter a partial Lyman limit break at ∼ 1950 Å since the neutral Hydrogen column density is not large enough for itto absorb all ionizing photons. Many absorption lines are produced by the DLA at z = 0.86 (C iv λλ1548, 1550, for example,is redshifted onto the red wing of the quasar’s Lyα emission line).

depth, τ , of the break is given by the productN(H i)σ, where the cross section for ionization ofHydrogen, σ = 6.3 × 10−18(Eγ/13.6 eV)−3 cm2,(and the flux is reduced by the factor e−τ ). Theenergy dependence of σ leads to a recovery ofthe Lyman limit break at higher energies (shorterwavelengths), unless N(H i) " 1017.2 cm−2 (seeFigure 1).

The curve of growth describes the relationshipbetween the equivalent width of an absorptionline, W , (the integral of the normalized profile)and its column density, N . Figure 3 shows thatfor small N(H i) the number of absorbed pho-tons, and therefore the flux removed, increasesin direct proportion to the number of atoms.This is called the linear part of the curve ofgrowth. As N is increased the line saturates sothat photons are only absorbed in the wings of

the lines; in this regime the equivalent width issensitive to the amount of line broadening (char-acterized by the Doppler parameter b), but doesnot depend very strongly on N(H i). This isthe flat part of the curve of growth. Finally, atN(H i) > 1020.3 cm−2, there are enough atomsthat the damping wings of the line become pop-ulated and the equivalent width increases as thesquare root of N(H i), and is no longer sensitiveto b.

In addition to the Lyα (1s → 2p) and higherorder (1s → np) Lyman series lines, quasar spec-tra also show absorption due to different ioniza-tion states of the various species of metals. Fig-ure 1 illustrates that the damped Lyα system atz = 0.86 that is responsible for the Lyα absorp-tion line at λobs = 2260 Å and a Lyman limitbreak at λobs = 1700 Å also produces absorp-

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Fig. 2.— The column density distribution of Lyαclouds, f(N(H i), roughly follows a power law over ten or-ders of magnitude; there are many more weak lines thanstrong lines. The column density regions for the three cat-egories of systems are shown: Lyα forest, Lyman limit,and damped Lyα. The term “Lyα forest” has at timesbeen used to refer to metal–free Hydrogen clouds, perhapsthose with N(H i) < 1016 cm−2, but now metals have beenfound associated with weaker systems down to the detec-tion limit.

tion at λobs = 2870 Å due to the presence ofC iv in the absorbing gas at that same redshift.Like many of the strongest metal lines seen inquasar spectra, C iv is a resonant doublet tran-sition due to transitions from 2S1/2 energy lev-els to the 2P1/2 and to the

2P3/2 energy levels.(The left superscript “2” represents the numberof orientations of the electron spin, the letter Sor P represents the total orbital angular momen-tum, L, and the right subscript represents the to-tal angular momentum, J .) Doublet transitionsare easy to identify. The dichotomy between restwavelength and redshift is resolved because theobserved wavelength separation of the doubletmembers increases as 1 + z.

Table 1 lists some of the metal lines that arecommonly detected for intervening absorptionsystems. Many of these are only strong enough tobe observable for quasar lines of sight that passthrough the higher N(H i) regions of galaxies.

Table 1: Common Transitions

Transition λrest [Å]LL . . . . . . . . . . . . . ∼ 912Lyγ . . . . . . . . . . . . 972.537Lyβ . . . . . . . . . . . . 1025.722Lyα . . . . . . . . . . . . 1215.670Si iv 1393 . . . . . . . 1393.755Si iv 1402 . . . . . . . 1402.770C iv 1548 . . . . . . . 1548.195C iv 1550 . . . . . . . 1550.770Fe ii 2382 . . . . . . . 2382.765Fe ii 2600 . . . . . . . 2600.173Mg ii 2796 . . . . . . 2796.352Mg ii 2803 . . . . . . 2803.531

2. History, Surveys, and Revolutionary

Progress in the 1990’s

The history of quasar absorption lines beganwithin a couple of years of the identification ofthe first quasar in 1963. In 1965, Gunn andPeterson considered the detection of flux blue-ward of the Lyα emission line in the quasar3C 9, observed by Schmidt, and derived a limiton the amount of neutral Hydrogen that couldbe present in intergalactic space. In that sameyear, Bahcall and Salpeter predicted that inter-vening material should produce observable dis-crete absorption features in quasar spectra. Suchfeatures were detected in 1967 in the quasarPKS 0237 − 23 by Greenstein and Schmidt, andin 1968 in PHL 938 by Burbidge, Lynds, andStockton. By 1969 many intervening systemshad been discovered, and Bahcall and Spitzerproposed that most with metals were producedby the halos of normal galaxies. As more data ac-cumulated, the sheer number of Lyα forest linesstrongly supported the idea that galactic and in-tergalactic gas, and not only material intrinsic tothe quasar, is the source of most quasar absorp-tion lines.

In the 1980’s many more quasar spectra were

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Fig. 3.— Illustration of the different regimes of the curve of growth. The middle panel shows the curve of growth forthe Lyα transition, relating the equivalent width, W , of the absorption profile to the column density, N(H i). The differentcurves represent four different values of the Doppler parameter: b = 13, 23, 53, and 93 km s−1. The upper panel showsabsorption profiles with Doppler parameter b = 23 km s−1 for the series of neutral hydrogen column densities N(H i) = 1012

– 1020 cm−2. The thick (thin) curves correspond to the filled (open) points on the b = 23 km s−1 curve of growth (middlepanel), starting at N(H i) = 1012 cm−2. For N(H i) < 1013 cm−2, known as the linear part of the curve of growth, theequivalent width does not depend on b. The lower left panel shows that, at fixed N(H i), the depth of the profile is smallerfor large b, such that the equivalent width remains constant. On the flat part of the curve of growth, profiles are saturatedand the equivalent width increases with b for constant N(H i). For N(H i) > 1020 cm−2, the profile develops damping wings,which dominate the equivalent width.

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obtained and many large statistical surveys ofthe different classes of absorption line systemswere published. The emphasis was to character-ize the number of lines per unit redshift, dN/dz,stronger than some specified equivalent widthlimit. With 4m–class telescopes [equipped withcharge coupled device (CCD) detectors] it waspossible to conduct surveys with a spectral res-olution of R ∼ 1000. The spectral resolution isdefined as R = λ/∆λ = c/∆v, so that R = 1000corresponds to 300 km s−1 or 5 Å at λ = 5000 Å.Separate surveys were conducted for Lyα lines,Mg ii doublets, C iv doublets, and also for Ly-man limit breaks, all as a function of redshift.The Lyα line is observable in the optical part ofthe spectrum for z > 2.2, Mg ii for 0.4 < z < 2.2,C iv for 1.7 < z < 5.0, and the Lyman limitbreak for z > 3. However, a break is also eas-ily identified in lower resolution space–based UVspectra, which extended Lyman limit surveys tolower redshift.

In order to consider the cross section of thesky covered by the different populations, it canbe assumed that absorption will be observed forall lines of sight within some radius of every lu-minous galaxy (> 0.05L∗K). (L

∗K represents the

Schechter luminosity, i.e. the transition betweenthe exponential and the power law forms of theluminosity function, and corresponds to a K–band absolute magnitude of MK = −25). Toexplain the observed dN/dz at z ∼ 1.5, thisradius would be 70 kpc for strong C iv (detec-tion sensitivity 0.4 Å), and 40 kpc for strongMg ii (detection sensitivity 0.3 Å) and also forLyman limit systems, implying that the lattertwo populations are in fact produced in the samegas. The higher N(H i) damped Lyα absorberswould be produced within 15 kpc of the centerof each galaxy, while the Lyα forest lines wouldrequire a considerably larger region, hundreds ofkpcs around each galaxy to produce a cross sec-tion consistent with the observed number of weaklines.

Up until the 1990’s, the focus of quasar ab-

sorption line work was to separately consider theproperties of the individual classes of absorbers(eg. Lyα forest or Mg ii absorbers). In the 1990’s,however, three different observational advancesled to recognition of the direct connections be-tween the different classes of quasar absorptionlines, and of direct associations with the popula-tion of galaxies:

1. Deep images of quasar fields could be ob-tained, and redshifts of the galaxies in the fieldcould be determined from low resolution spec-tra. Steidel found that whenever Mg ii absorp-tion with Wr(Mg ii) > 0.3 Å is observed, a lu-minous galaxy (LK > 0.06L∗K) is found withinan impact parameter of 38h−1(L/L∗K)

−0.15 kpcwith a redshift coincident with that determinedfrom the absorption lines. Also, it is rare to finda galaxy within this impact parameter that doesnot produce Mg ii absorption. There appears tobe a one–to–one correspondence between strongMg ii absorption and luminous galaxies. TheMg ii absorbing galaxies span a range of morpho-logical types.

2. The High Resolution Spectrograph on theKeck I 10-meter telescope made it possible to ob-tain quasar spectra at a resolution of R = 45, 000,which corresponds to ∼ 6 km s−1. The previ-ous surveys with resolution of order hundreds ofkm s−1 identified absorption due to entire galax-ies and their environments. With 6 km s−1 res-olution it became possible to resolve structurewithin a galaxy: the clouds in its halo, the in-terstellar medium of its disk, and the satellitesand infalling gas clouds in its environment. Fig-ure 4 is a dramatic illustration of this contrast forthe Mg ii absorber at z = 0.93 toward the quasarPG 1206 + 459.

3. The Faint Object Spectrograph (FOS) onthe Hubble Space Telescope provided resolutionR ∼ 1000 in the UV, from 1400–3300 Å. Obser-vations of Lyα forest clouds could be extendedfrom z = 2.2 down to the present epoch. Fur-thermore, absorption from a given galaxy couldbe observed in numerous transitions; if Mgii was

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Fig. 4.— Dramatic demonstration of gains due to high resolution spectroscopy of the Mg ii doublet. The top panel isa R = 3000 spectrum of PG1206 + 459. The doublet that is apparent at an observed wavelength of ∼ 5400 Å is due toMg ii absorption from a system at z = 0.927. The middle panel shows the remarkable kinematic structure that is revealedat the resolution (R = 45, 000) of the Keck/HIRES spectrograph of the same quasar. The 2796 Å transition is resolvedinto multiple components (5583–5592 Å), which also appear in the 2803 Å transition (5396–5406 Å). This system can beseparated in two “clusters” of clouds, labeled “A” and “B”. Another weaker Mg ii doublet is observed at 5409 and 5423 Å,from a system at z = 0.934 Å, labeled with a “C”. The solid line through these complex Mg ii profiles is the result of multipleVoigt profile fitting, with a cloud centered on each of the ticks drawn above the spectrum. The lower panel shows theC iv doublets associated with the same three systems, observed with the Faint Object Spectrograph on HST, but at muchlower resolution (R = 1300). The C iv is in three different concentrations around the three systems “A”, “B”, and “C”. TheC ivλ1550 transition from system A is blended with the C ivλ1548 transition from system B. The C iv equivalent width is toolarge for this absorption to be produced by the same phase of gas that produces the Mg ii cloud absorption. The maximumabsorption that can arise in the Mg ii phase is given by the dotted line; a plausible model with a kinematically broader C ivphase yields the solid curve.

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observed in the optical, the Lyman series andC iv could be studied in the UV (see Figure 4).With information on transitions with a range ofionization states, consideration of the degree ofionization (related to the gas density and the in-tensity and shape of the ionizing radiation field)and the multiple phase structure of galactic gasbecame possible.

No longer is analysis of absorption lines inquasar spectra an esoteric subject. It has devel-oped into a powerful tool to be used in the studyof galaxy evolution (eg. similar to imaging thestellar components of the galaxies). At least inprinciple, quasar spectra can be used for an unbi-ased study of the gaseous environments of galax-ies from the present back to the highest redshiftsat which quasars are observed. Gas structuressmaller than 1 M# can be detected if they areintercepted by the quasar line of sight, irrespec-tive of whether they emit light. Through the toolof quasar absorption lines, proto–galactic struc-tures and low surface brightness galaxies can bestudied as well as high luminosity galaxies.

3. Developing Physical Intuition

With high resolution spectra of quasars, it ispossible to consider the physical conditions ofthe gaseous structures that produce absorption.However, it is challenging to separate the vari-ous effects that “shape” the spectral features inthe different chemical transitions. The absorp-tion profiles observed for the different chemicaltransitions are determined by a combination ofthe spatial distribution of material along the lineof sight, its bulk kinematics, temperature, metal-licity, and abundance pattern. The ionizationstructure is influenced by gas densities and bythe UV radiation field, which is a combinationof the extragalactic background radiation due tothe accumulated effect of quasars and stellar pho-tons escaped from galaxies (and corrected for ab-sorption by the intergalactic medium).

The shape of an absorption line can be mod-

eled with a Voigt profile, which is a combinationof the natural, quantum mechanical Lorentzianbroadening and the Gaussian broadening causedby the thermal and turbulent motions in the gas.Several Voigt profiles can be blended together toform an overall complex absorption feature (seeFigure 4). The “width” of a single Voigt profileis characterized by the Doppler parameter, b (ex-pressed in velocity units and related to the Gaus-sian σ by b = 21/2σ). Physically, the Dopplerparameter is the sum of thermal and turbulentcomponents, b2tot = 2kT/m + b

2turb, where T is

the temperature of the gas, and m is the mass ofan atom.

3.1. Kinematic Models

Two of the simplest types of organized kine-matics in galaxies are illustrated in Figure 5:clouds distributed in a rotating disk, and radialinfall of clouds in a spherical distribution. Here,Mg ii absorbers are used as an example, but thesame kinematic arguments would apply to othertransitions. For radial infall, clouds can be dis-tributed over the range of velocities, with a ten-dency for a “double peak” from material that isredshifted and blueshifted but with a consider-able amount of variation if there are typicallyseveral discrete clouds along the line of sight.A rotating disk with a vertical velocity disper-sion characteristic of a spiral galaxy disk (10–20 km s−1) will have clouds superimposed in ve-locity space, and an overall kinematic spread oftens of km s−1. Strong Mg ii absorption has beenfound to arise along nearly all lines of within∼ 40 kpc of normal galaxies (i.e. the covering fac-tor is nearly unity within that radius). The largevariety of kinematics evident in Mg ii absorptionprofiles is, in fact, consistent with a superposi-tion of disk and radial infall (halo) motions, andnot with just one or the other. In addition tothese simple, toy models, insights can be gleanedby passing lines of sight through the structuresin cosmological N–body/hydrodynamic simula-tions. In a few studies, metals have been added

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Fig. 5.— Illustrations of two simple kinematic models are shown in the top panel. To the left, the model is radial infallof clouds to the center of a sphere, with constant velocity. The line of sight passes through five clouds, which leads to fivedifferent absorption features (for a single transition) in the quasar spectrum. Two of the features are blueshifted relativeto the standard of rest of the absorbing galaxy, and the other three are redshifted. The absorption features from a radialinfall model can be spread over a velocity of 100–200 km s−1, typical of the velocity dispersion of a galaxy halo. To theright, a rotating disk model is illustrated. In this case all the “clouds” along the line of site have a component of motionthat is redshifted, and they tend to be clustered together in velocity space, with typical spread of 20-60 km s−1. The lowerpanel shows a sample of 0.4 < z < 1.4 Mg ii absorption profiles observed with the Keck/HIRES spectrograph at R = 45, 000,corresponding to a resolution of ∼ 6 km s−1. The solid lines through these data are Voigt profile fits and the ticks drawnabove the spectrum represent the cloud velocities. Some of these profiles are consistent with the kinematics of a rotatingdisk, and others with radial infall kinematics. However, to explain the full ensemble of profiles a model combining these twobasic types of kinematics is needed.

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uniformly throughout the simulation box andphotoionization models used to predict the ab-sorption expected from different structures. Thisis especially important for establishing the kine-matics that would be observed from the processof structure formation at high redshifts.

3.2. Photoionization Models

Consider a cloud of material, modeled by aplane parallel slab with a certain total columndensity of Hydrogen, N(H) = N(H i) + N(H ii),and with a constant total number density nH =n(H i)+n(H ii) along the line of sight. The cloudis also characterized by its metallicity, Z, whichis the ratio of Fe/H expressed relative to the so-lar value, Z#, and by an abundance pattern (theabundance ratios of all other elements to Fe).The degree of ionization in the gas depends uponthe intensity and shape of the spectrum of ioniz-ing radiation. The intensity is characterized bythe ionization parameter, U = nγ/nH , which isthe ratio of the number density of photons at theLyman edge to the number density of Hydrogen(nH = ne, where ne is the total number densityof electrons). The larger the value of U , the moreionized the gas. Collisional ionization can also bean important process for some absorption sys-tems with gas at high temperatures (hundreds ofthousands of degrees). Photoionization equilib-rium models typically yield temperatures of tensof thousands of degrees.

Once the metallicity, abundance pattern, ion-ization parameter, and spectral shape are spec-ified the equations of radiative transfer can besolved to find the column densities of all the dif-ferent ionization states of various chemical ele-ments. Figure 6 illustrates, for N(H i) = 1016

and 1019 cm−2, the dependence of column densi-ties of various transitions on the ionization pa-rameter, U . For optically thin gas [N(H i) <1017.2 cm−2], the column density ratios of thevarious metal transitions are not dependent onthe overall metallicity, i.e. the curves shift verti-cally in proportion to Z. For optically thick gas,

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Fig. 6.— Photoionization model predictions of the col-umn densities of Mg ii, Fe ii, and C iv as a function ofthe ionization parameter (the ratio of ionizing photons tothe electron number density in the gas). The spectrumincident on the cloud, represented by a constant den-sity slab, is the “Haardt–Madau” spectrum (attenuatedspectrum due to integrated effect of quasars and younggalaxies). The predicted column densities are presentedin two series of models with N(H i) = 1016 cm−2 and withN(H i) = 1019 cm−2, the optically thin and optically thickcases. For both, the metallicity is fixed at 10% of the so-lar value. For the optically thin case, the column densitiesscale with metallicity, i.e. the ratios remain constant, butfor the optically thick case the situation is more complex.

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ionization structure develops, with an outer ion-ized layer around a neutral core, and there is nosimple scaling relation with metallicity.

In practice, if we assume that a cloud has asimple, single phase structure, the ratios of thecolumn densities can be used to infer the ioniza-tion parameter, which relates to the density ofthe gas. However, the abundance pattern candiffer from the solar abundance pattern becauseof differing degrees of depletion onto dust, orbecause of different processing histories. Mostof the so–called α particle nuclei (such as Mgand Si) are synthesized primarily by Type II su-pernovae during the early history of a galaxywhen most massive stars form and quickly evolveto reach their end states. On the other hand,the Fe–group elements are primarily producedby Type Ia supernovae, and therefore build upover a longer timescale. In the basic picture ofgalaxy evolution, the halo stars are formed early,have been enriched only by Type II supernova,and therefore are α–element enhanced. Youngerdisk stars have incorporated also the Type Iaprocessed material and therefore have relativelylarger Fe–group abundances. Ideally, several dif-ferent ionization states of the same chemical ele-ment are observed so that there is no ambiguitybetween the ionization parameter and the abun-dance pattern, but this has generally not yet beenpossible because of limited wavelength coverageat high resolution.

Examples of the variation of column densityratios with velocity in two absorption systemsare shown in Figures 7 and 8. In Figure 7,N(Fe ii)/N(Mg ii) varies by an order of magni-tude over the four components in the z = 1.325system toward the quasar Q0117+213. This rep-resents a variation of an order of magnitude inthe ionization parameter (10−4 < U < 10−3), oran order of magnitude variation in the abundancepattern. Figure 8 is a very unusual system withtwo clouds separated by only 20 km s−1 in veloc-ity, one of which has a Silicon to Aluminum ra-tio similar to the Milky Way ISM, and the other

which requires a significant enhancement of Alu-minum.

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Fig. 7.— HIRES/Keck Fe ii and Mg ii absorption profilesfor the z = 1.325 system in the spectrum of the quasarQ0117 + 213. The six clouds in this system show a rangeof more than an order of magnitude in N(Fe ii)/N(Mg ii),given below each cloud in the lower panel. These varia-tions could be due to cloud to cloud variations of ioniza-tion parameter (density) or of abundance pattern withinthe system.

4. Multiphase Conditions

The gaseous component of the the Milky Wayand nearby galaxies have phase structure (i.e.spatial locations with different densities and/ortemperatures). Examples are the disk/halo inter-face (Galactic coronae) and the cold, warm, andhot phases of the interstellar medium. From pho-toionization models, it is not usually possible togenerate absorption that is simultaneously con-sistent with all observed chemical transitions fora given system. For example, in single cloud Mg iisystems, Figure 6 (with N(H i) = 1016 cm−2)shows that if Fe ii is detected at a similar columndensity to Mg ii, the ionization parameter must

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Fig. 8.— An unusual Aluminum–rich cloud is apparentin the z = 1.93 system toward the quasar Q1222+228, andit is close in velocity space to a normal (relative to Galacticclouds) cloud which has detected Si ii. Note the differentkinematic structure in the higher ionization transitions.The excess of Al ii and Aliii in the cloud at v = 9 km s−1

is best explained by an abundance pattern variation, sinceSi ii and Al ii are transitions with very similar ionizationstates.

be small, and Wr(C iv) cannot be large. Manysystems have C iv absorption which exceeds thislimit and requires a higher ionization (lower den-sity) phase; generally, this phase must have struc-ture over a large velocity range (a large “effec-tive” Doppler parameter). The z = 0.93 systemtoward the quasar PG 1206+459 is another casethat requires multiphase structure. The observedC iv profile in Figure 4 is much too strong for thisabsorption to arise in the same clouds that pro-duce the Mg ii, even if their ionization parametersare pushed to the largest values consistent withthe data.

5. Statistics, Evolution, and Interpreta-

tion

Future quasar absorption line studies will com-bine insights gained from detailed analyses of in-dividual systems with conclusions drawn fromthe large statistical samples assembled over cos-mic time. Evolution of the ensemble of absorp-tion profiles generated by the universal collectiveof intervening structures is a result of the com-bined effects of numerous processes. These in-clude growth of structure, star formation, mor-phological evolution of galaxies, galaxy mergers,and changes in the extragalactic background ra-diation. Here, we summarize the best presentstatistical data and likely interpretations for thedifferent classes of absorbers. The number oflines per unit redshift for various populations ofabsorbers is represented by a power law dN/dz ∝(1 + z)γ . For a universe with only the cosmo-logical evolution due to expansion, γ = 1.0 fordeceleration parameter q0 = 0 and γ = 0.5 forq0 = 0.5.

5.1. Lyα Forest

The Lyα forest evolves away dramaticallyfrom high to low redshift, as is strikingly clearfrom the spectra of z ∼ 3 and z ∼ 1 quasarsin Figure 9. The evolution of the Lyα lines withWr(Lyα) > 0.3 Å can be characterized by a dou-

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z=1

Fig. 9.— Illustration of structure evolution of intergalactic gas from high to low redshift. The upper spectrum of a z = 3.6quasar is a Keck/HIRES observation, while the lower spectrum is a FOS/HST observations of a z = 1.3 quasar. Higherredshift quasars show a much thicker forest of Lyα lines. Slices through N–body/hydrodynamic simulation results at thetwo epochs z = 3 and z = 1 are shown in the right–hand panels. Three contour levels are shown: 1011 cm−2 (dotted lines),1012 cm−2 (solid lines) and 1013 cm−2 (thick solid lines). Evolution proceeds so that the voids become more empty so thateven the low column density material is found in filamentary structures at low redshifts.

ble power law with γ ∼ 2 for 1.8 < z < 4.5and γ ∼ 0.2 for z < 1.8. Help in understand-ing the physical picture has come from sophis-ticated N–body/hydrodynamic simulations thatincorporate the gas physics and consider cosmo-logical expansion of the simulation box. The dy-namical evolution of the H i gas can be describedas outflow from the centers of voids to their sur-rounding shells, and flows along these sheets to-ward their intersections where the densest struc-tures form. This picture is consistent with ob-servational determinations of the “sizes” of Lyαstructures. It is difficult to obtain direct mea-surements of sizes except in some special casesto use “double lines of sight”, close quasar pairs,either physical or apparent due to gravitationallensing. If the spectra of the two quasars bothhave a Lyα absorption line at the same wave-

length that implies a “structure” which coversboth lines of sight. From these studies, it is foundthat “structures” are at least hundreds of kpc inextent.

At redshifts z = 5 to z = 2 dN/dz for Lyα for-est absorption is quite large, but it is decliningvery rapidly over that range. This dramatic evo-lution in the number of forest clouds is mostlydue to the expansion of the universe, with amodest contribution from structure growth. Atz < 2, the extragalactic background radiationfield is falling, and Lyα structures are becom-ing more neutral. Therefore, the more numerous,smaller N(H) structures are observed at a largerN(H i) and this will counteract the effect of ex-pansion, thus slowing the decline of the forest.

The high redshift Lyα forest was once thought

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to be primordial material, but in fact it is ob-served to have a metallicity of 0.1% solar, evenat z = 3. For N(H i) < 1014 cm−2, the expectedN(C iv) would be below the detection thresholdsof current observations, so truly pristine mate-rial still eludes us. Perhaps it does not exist.To spread metals all through the intergalacticmedium may have required a “pre–galactic” pop-ulation of stars at z > 10 that polluted all ofintergalactic space.

5.2. Lyman Limit and Metal Line Sys-

tems

The dN/dz of Lyman limit systems is consis-tent with that of strong Mg ii absorbers [withWr(Mg ii) > 0.3 Å] over the redshift range forwhich both have been observed, 0.4 < z < 2.2.For Wr(Mg ii) > 0.3 Å, γ = 1.0 ± 0.1, consistentwith no evolution. For even stronger Mg ii sys-tems [Wr(Mg ii) > 1 Å], dN/dz increases moredramatically with z, with γ = 2.3 ± 1.0.

The number of Mgii systems (equivalent widthdistribution) continues to increase down to thesensitivity of the best surveys, Wr(Mg ii) > 0.02 Å,such that dN/dz = 2.7 ± 0.15 at z ∼ 1. The“weak” Mg ii absorbers are therefore more com-mon than the strong systems [Wr(Mg ii) > 0.3 Å]known to be associated with luminous galaxies.Unlike the strong Mg ii absorbers, the weak Mg iiabsorbers are sub-Lyman limit systems (they donot have Lyman limit breaks), and no galaxieshave been identified at the redshift of absorp-tion. Yet, photoionization models indicate thatthe metallicities of these weak absorbers are atleast 10% of the solar value, and in some casescomparable to solar. They are a varied popula-tion: some have relatively strong Fe ii while oth-ers have no Fe ii detected, and some have strongC iv that requires a separate phase while othershave no C iv detected. Those with strong Fe iiare constrained to be smaller than 10 pc (theionization parameter must be small and ne largeas can be seen in Figure 6). Also, since Fe isproduced primarily by Type Ia supernovae they

must be enriched by a relatively old stellar pop-ulation. Those with weaker or undetected Fe iicould be larger (kpcs or tens of kpcs) and pos-sibly enriched by Type II supernovae. Candi-date environments that could be traced by weakMg ii absorption are: remnants of pre–galacticstar clusters formed in mini–halos at z > 10, su-per star clusters formed in interactions, tidallystripped material, low surface brightness galax-ies, and ejected or infalling clouds (analogous tothe Milky Way high velocity clouds).

The evolution of dN/dz for C iv absorbers canbe studied in the optical for high redshifts. ForW (C iv) > 0.4 Å and z > 1.2, the number de-creases with increasing z, as γ = −2.4 ± 0.8. Inthis same interval, the number of Lyman limitsystems is still increasing with redshift, withγ = 1.5 ± 0.4. This implies that the dramaticevolution in the number of C iv systems is eitherdue to a change in metallicity or a change in ion-ization state. The dN/dz for C iv systems peaksat intermediate z and declines, consistent withno evolution until the present. Combining opti-cal and UV data, C iv and Mg ii have been com-pared at 0.4 < z < 2.2. The fraction of systemswith large Wr(C iv)/Wr(Mg ii) decreases rapidlywith decreasing redshift; there is a shift toward“lower ionization systems”.

It is important to consider that the H i, Mg ii,and C iv absorption do not always arise in thesame phase. It is possible that the C iv in manyz ∼ 1 Mg ii absorption systems arises in a phasesimilar to the Galactic coronae. If the origin ofthis phase is related to star–forming processes inthe disk, then it might be expected to diminishbelow z = 1.2 since the peak star formation rateis passed.

Another important trend is the fact that thevery strongest Mg ii absorbers evolve away fromz = 2 until the present. If we study the kinematicstructure of these objects, we find that they com-monly have a “double” structure, with two sepa-rate kinematic regions in the Mg ii profile. Theseobjects also have strong C iv which also has sepa-

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rate components around the two Mg ii regions inthe “double” structure. The C iv does not ariseprimarily in the individual Mgii clouds, nor is itin a smooth, “common halo” structure that ex-tends in velocity space around the entire Mg iiprofile. As more data are collected on the kine-matic structure of various transitions in these“double” systems, it will be interesting to con-sider the hypothesis that galaxy pairs in the pro-cess of merger are responsible. The number ofthese is thought to have been dramatically largerin the past.

5.3. Damped Lyα Systems

The N(H i) > 1020.3 cm−2 systems are of par-ticular interest because it is possible to observemany different chemical elements (such as Zn,Cr, Fe, Mn, and Ni) in these objects back tohigh redshift. Metallicities and abundance pat-terns can be studied and compared to those ofold stellar populations in the Milky Way. Backto z = 3, the metallicity in DLAs, as measuredby the undepleted element Zinc, is about 10% ofthe solar value, but it may decline at z > 3. Theidentity of sites responsible for DLAs at high z re-mains controversial, but they do contain most ofthe neutral Hydrogen in the universe, from whichmost of its stars form. The kinematic structureof the absorption profiles of neutral and low ion-ization species is consistent with the rotation ofa thick disk, so that it is possible that these arethe z = 3 progenitors of normal spiral galaxies.However, this signature is not unique. It couldalso be the consequence of directed infall in anhierarchical structure formation scenario. Thehigher ionization species show complex kinemat-ics which vary in relation to those of the lowerionization gas; in some systems they appear totrace relatively similar structure, and in othersthere are clearly several different phases.

At low redshift, many of the galaxies that areresponsible for the DLA absorption can be di-rectly identified. These galaxies are a heteroge-neous population. They are not just the most lu-

minous galaxies, but include dwarf and low sur-face brightness galaxies, and even cases whereno galaxy has been identified to sensitive limits.Damped Lyα absorption does not trace the mostluminous objects, but rather it traces the largestneutral gas reservoirs. An additional selectioneffect may be important. The most dust–richgalaxies that have the potential to produce DLAabsorption could produce enough extinction thattheir background quasars will not be included inquasar surveys. In this way, the population ofDLAs that are actually observed could be signif-icantly biased against dusty galaxy hosts.

6. Future Prospects

The next decade will see the synthesis of thevarious techniques for the study of galaxy evo-lution, through their stars and through theirgas. Higher resolution quasar spectra will beobtained in the ultraviolet (with the Space Tele-scope Imaging Spectrograph (STIS) and with theCosmic Origins Spectrograph (COS) on the HST,and later, hopefully, with a larger UV space tele-scope). It will then be possible to conduct a sys-tematic analysis of the relationships between thedifferent ionization species that trace the differ-ent phases of gas in 0.4 < z < 1.5 galaxies. Inthis redshift regime, comparisons to the detailedmorphological structure and orientations of theabsorbing galaxies is possible from HST images.

Invaluable insights into the origin of quasarabsorption lines have been gleaned from absorp-tion studies of nearby galaxies, for which it ispossible to directly observe the processes that areinvolved. Making more observations of this typewill be possible by discoveries of bright quasarsthat fall behind nearby galaxies. The discoveriesof quasars in large surveys will also include mul-tiple lines of sight behind distant absorption linesystems which can be used to produce 3–D mapsof the structures.

The interstellar medium of the Milky Wayshows structure on sub–pc scales, and absorp-

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tion features can only be resolved with resolution< 1 km s−1. Such a resolution will soon be avail-able on 8m–class telescopes. This is importantfor separating blends and for looking for metallic-ity, ionization, and abundance pattern gradientsalong the line of sight.

The key low ionization transitions of Mg ii andFe ii are shifted into the near–IR region of thespectrum for z > 2.5. Very soon, near–IR quasarspectra will be obtained at relatively high resolu-tion (∼ 20 km s−1). Also, IR–imaging, narrow–band techniques, and multi–object spectroscopyin the near–IR should provide much more infor-mation about absorbing galaxies at higher red-shifts. This will extend evolutionary studies backto an epoch at which formation processes may becontributing significantly to evolution.

7. Bibliography

Articles in review journals:

Rauch M 1998 The Lyman Alpha Forest in theSpectra of QSOs ARAA 36 267

Churchill C W and Charlton J C 2000 Mg ii Ab-sorbers: A Review PASP in press

Conference proceedings:

Blades J C, Turnshek D A, and Norman C 1988QSO Absorption Lines: Probing the Universe,

Proceedings of the QSO Absorption Line Meet-

ing, Baltimore 1987 (Cambridge: CambridgeUniversity Press)

Meylan G 1995 QSO Absorption Lines: Proceed-ings of the ESO Workshop, Munich 1994 (Berlin:Springer)

Petitjean P and Charlot S 1997 Structure andEvolution of the Intergalactic Medium from QSO

Absorption Lines (Paris: Editions Frontiéres)

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