Over the last quarter century, studies of the circumgalactic medium (CGM) have evolved from small, isolated cottage-industry efforts to a few dozen factory-scale assembly-line collaborations. The advent and continued development of large galaxy surveys, the refinement of photometric redshifts, and the honing of color selection of quasars have all combined to yield more than a million object-searchable catalogs for building large samples of galaxy-quasar pairs on the sky. Though the largest body of work has focused on low- and intermediate-redshifts, where detailed galaxy properties can be measured, wholesale studies of the CGM have now reached redshifts of 4 using Lyman break galaxies (LBGs) and the stacking of the spectra of thousands of Lyman alpha emitters. In this chapter, we provide an overview of CGM studies with a focus on sample building and experimental approaches and techniques. The three main types of survey strategies are discussed. Concepts such as the characterization of CGM absorption properties as a function of impact parameters, covering fractions, and galaxy-absorber morphokinematic and morphospatial analysis are presented.

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.

Relativistic collision kinematics is examined. Relativistic and non-relativistic collisional processes in plasmas are treated in detail.