QSO Absorption Line Systems as a Probe of Galaxies Like the Milky Way

Quasar absorption lines provide detailed information on the chemical, kinematic, and ionization conditions in galaxies and their environments, and provide a means for studying the evolution of these conditions back to the epoch of the first quasars. Among the collection of absorbing structures along the lines of sight to quasars there is an evolutionary sequence of galaxies that represent predecessors of the Milky Way and provide a direct view of its history. Absorption spectra of lines of sight through the Milky Way and through nearby galaxies reveal a variety of chemical species, ionization conditions, and kinematic substructures. These absorption profiles are produced by low density gas distributed in rotating disks, high velocity halo clouds, satellite galaxies and their debris, superbubbles, and other sub-galactic gaseous fragments. Guided by knowledge gained by studying nearby galaxies, we are developing interpretations of the variety of observed absorption signatures. Images of .4 < z < 1 galaxies responsible for MgII absorption also allow us to explore the statistical connections between the galaxy properties and their gaseous content. Quasar absorption lines are fast becoming a powerful evolutionary probe of gaseous conditions in the Milky Way.