THE KINEMATIC COMPOSITION OF MG-II ABSORBERS

The study of galaxy evolution using quasar absorption lines requires a n understanding of what components of galaxies and their surroundings are contributing to the absorption in various transitions. This paper considers the kinematic composition of the class of 0.4 < z < 1.0 Mg I I absorbers, particularly addressing the question of what fraction of this absorption is produced in halos and what fraction arises from gal axy disks. We design models with various fractional contributions from radial infall of halo material and from a rotating thick disk compone nt. We generate synthetic spectra from lines of sight through model ga laxies and compare the resulting ensembles of Mg rr profiles with the 0.4 less than or equal to z less than or equal to 1.0 sample observed with the Keck Telescope HIRES. We apply a battery of statistical tests and find that pure disk and pure halo models can be ruled out, but th at various models with rotating disk and infall/halo contributions can produce an ensemble that is nearly consistent with the data. A discre pancy in all models that we considered requires the existence of a kin ematic component intermediate between halo and thick disk. The variety of Mg II profiles can be explained by the gas in disks and halos of g alaxies being not very much different than galaxies in the local Unive rse. In any one case, there is considerable ambiguity in diagnosing th e kinematic composition of an absorber from the low-ionization high-re solution spectra alone. Future data will allow galaxy morphologies, im pact parameters and orientations, Fe II/Mg II of clouds, and the distr ibution of high-ionization gas to be incorporated into the kinematic a nalysis. Combining all these data will permit a more accurate diagnosi s of the physical conditions along the line of sight through the absor bing galaxy.