The depths, widths, and shapes of absorption lines are the code of optical depth profiles. Line depth is the amplitude of the optical depth, which is absorber column density. Line width and shape mirror the total cross section. This is the atomic cross section convolved with a wavelength redistribution function, usually a Gaussian attributable to thermal Doppler broadening. The resulting optical depth profile is a Voigt function. In this chapter, we quantitatively described Voigt profiles in detail. The total absorption is the equivalent width and its functional dependence on column density and Doppler broadening is called the curve of growth. Expressions are derived for its three major regimes: the linear, flat, and damped “parts.” The measured equivalent width increases with increasing absorption redshift, and this must be calibrated out. Inverting absorption line profiles yields apparent optical depth (AOD) profiles, which can be converted into integrable column density profiles. We also describe how to compute the covering factor from doublets showing signs of partial covering and conclude with an in-depth discussion of Lyman-limit ionization breaks from optically thick absorbers.

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

Thechemical composition of stars is a central issue in the study of stellar photospheres.What observations are needed, how are they to be processed, and what is the nature of the results?Other topics include variations in metallicity with location in the galaxy and with time.Thebehavior of lithium with temperature and time is particularly intriguing.