eaa.iop.orgDOI: 10.1888/0333750888/2366 Quasistellar Objects: Intervening Absorption LinesJane Charlton, Chris Churchill FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:37:03 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsQuasistellar Objects: Intervening Absorption LinesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSQuasistellar Objects: InterveningAbsorption LinesEvery parcel of gas along the line of sight to a distantQUASAR will selectively absorb certain wavelengths ofcontinuum light of the quasar due to the presence ofthe various chemical elements in the gas. Through theanalysis of these quasar absorption lines we can study thespatial distributions, motions, chemical enrichment andionization histories of gaseous structures fromREDSHIFTfive until the present. This includes the GAS IN GALAXIES ofall morphological types as well as the diffuse gas in theINTERGALACTIC MEDIUM.Basics of quasar spectraFigure 1 illustrates many of the common features of aquasar spectrum. The relatively flat quasar continuumand broad emission features are produced by the quasaritself (near the black hole and its accretion disk). Insome cases, gas near the quasar central engine alsoproduces ‘intrinsic’ absorption lines, most notably Ly α,and relatively high ionization metal transitions such asC IV, N V, and O VI. These intrinsic absorption lines canbe broad (thousands or even tens of thousands of km s−1in which case the quasar is called a broad absorptionline (BAL) QSO; seeQUASISTELLAR OBJECTS: INTRINSIC AGNABSORPTION LINES), or narrow (tens to hundreds of km s−1).However, the vast majority of absorption lines in atypical quasar spectrum are ‘intervening’, produced bygas unrelated to the quasar that is located along the line ofsight between the quasar and the Earth.Astructure along the line of sight to the quasar can bedescribed by its neutral hydrogen column density, N(H I),the number of atoms per cm2. N(H I) is given by theproduct of the density of the material and the pathlengthalong the line of sight through the gas. Each structurewill produce an absorption line in the quasar spectrumat a wavelength of λobs= λrest(1+zabs), where zabsis theredshift of the absorbing gas and λrest= 1215.67 Å is therest wavelength of the Ly α transition. Since zabs<zQSO,the redshift of the quasar, these Ly α absorption lines forma ‘forest’ at wavelengths blueward of the Ly α emission.The region redward of the Ly α emission will be populatedonly by absorption through other chemical transitionswith longer λrest. Historically, absorption systems withN(H I) < 1017.2cm−2have been called LYMAN ALPHA FORESTlines, those with 1017.2<N(H I) < 1020.3cm−2are Lymanlimit systems, and those with N(H I) > 1020.3cm−2aredamped Ly α systems (seeLYMAN ALPHA ABSORPTION: THEDAMPED SYSTEMS).Thenumber of systemsper unit redshiftincreasesdra-matically with decreasing column density, as illustrated inthe schematic diagram in figure 2. Lymanlimit systems aredefined by a sharp breakin the spectrum due to absorptionof photons capable of ionizing H I, i.e. those with energiesgreater than 13.6 eV. The optical depth, τ , of the break isgiven by the product N(H I)σ , where the cross section forionization of hydrogen, σ = 6.3× 10−18(Eγ/13.6eV)−3cm2,(and the flux is reduced by the factor e−τ). The energydependence of σ leads to a recovery of the Lyman limitbreak at higher energies (shorter wavelengths), unlessN(H I)  1017.2cm−2(see figure 1).The curve of growth describes the relationshipbetween the equivalent width of an absorption line, W (theintegral of the normalized profile), and its column density,N. Figure 3 shows that for small N (H I) the numberof absorbed photons, and therefore the flux removed,increases in direct proportion to the number of atoms.This is called the linear part of the curve of growth. AsN is increased, the line saturates so that photons areonly absorbed in the wings of the lines; in this regimethe equivalent width is sensitive to the amount of linebroadening (characterized by the Doppler parameter b),but does not depend very strongly on N (H I). This is theflat part of the curve of growth. Finally, at N (H I) >1020.3cm−2, there are enough atoms that the dampingwings of the line become populated and the equivalentwidth increases as the square root of N(H I), and is nolonger sensitive to b.In addition to the Ly α (1s → 2p) and higher-order(1s → np) Lyman series lines, quasar spectra also showabsorption due to different ionization states of the variousspecies of metals. Figure 1 illustrates that the dampedLy α system at z = 0.86 that is responsible for the Ly αabsorption line at λobs= 2260 Å and a Lyman limit breakat λobs= 1700 Å also produces absorption at λobs= 2870 Ådue to the presence of C IV in the absorbing gas at thatsame redshift. Like many of the strongest metal lines seenin quasar spectra, C IV is a resonant doublet transition dueto transitions from2S1/2energy levels to the2P1/2and tothe2P3/2energy levels. (The left superscript ‘2’ representsthe number of orientations of the electron spin, the letterS or P represents the total orbital angular momentum,L, and the right subscript represents the total angularmomentum, J .) Doublet transitions are easy to identify.The dichotomy between rest wavelength and redshift isresolved because the observed wavelength separation ofthe doublet members increases as 1+z. Table 1 lists some ofthe metal lines that are commonly detected for interveningabsorption systems. Many of these are only strong enoughto be observable for quasar lines of sight that pass throughthe higher N(H I) regions of galaxies.History, surveys and revolutionary progress in the1990sThe history of quasar absorption lines began within acouple of years of the identification of the first quasarin 1963. In 1965, Gunn and Peterson considered thedetection of flux blueward of the Ly α emission line inthe quasar 3C 9, observed by Schmidt, and derived alimit on the amount of neutral hydrogen that could bepresent in intergalactic space. In that same year, Bahcalland Salpeter predicted that intervening material shouldproduce observable discrete absorption features in quasarspectra. Such features were detected in 1967 in the quasarPKS 0237 − 23 by Greenstein and Schmidt, and in 1968 inCopyright © Nature Publishing Group 2001Brunel