The Kinematic Composition of Mg II Absorbers

The study of galaxy evolution using quasar absorption lines requires an 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 II absorbers, particularly addressing the question of what fraction of this absorption is produced in halos and what fraction arises from galaxy disks. We design models with various fractional contributions from radial infall of halo material and from a rotating thick disk component. We generate synthetic spectra from lines of sight through model galaxies and compare the resulting ensembles of Mg II profiles with the 0.4 ≤ z ≤ 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 that various models with rotating disk and infall/halo contributions can produce an ensemble that is nearly consistent with the data. A discrepancy in all models that we considered requires the existence of a kinematic component intermediate between halo and thick disk. The variety of Mg II profiles can be explained by the gas in disks and halos of galaxies being not very much different than galaxies in the local Universe. In any one case, there is considerable ambiguity in diagnosing the kinematic composition of an absorber from the low-ionization high-resolution spectra alone. Future data will allow galaxy morphologies, impact parameters and orientations, Fe II/Mg II of clouds, and the distribution of high-ionization gas to be incorporated into the kinematic analysis. Combining all these data will permit a more accurate diagnosis of the physical conditions along the line of sight through the absorbing galaxy.