THE KINEMATICS OF INTERMEDIATE-REDSHIFT GASEOUS GALAXY HALOS

We present high-resolution (FWHM almost-equal-to 7-35 km s-1) spectroscopy of the absorption lines associated with four intermediate-redshift galaxies: the z = 0.5240 absorber toward the BL Lac object 0235 + 164, the z = 0.7260 absorber toward the QSO 0453-423, the z = 0.3950 absorber toward the QSO 1229-021, and the z = 0.4298 absorber toward the QSO 2128-123. These observations are combined with existing spectroscopy of the redshifted 21 cm absorption lines toward 0235 + 164 and 1229-021. The absorption lines toward 0235 + 164 and 1229-021 are complex and span total velocity intervals of roughly 250 km s-1. The absorption lines toward 0453-423 are moderately complex and span a total velocity interval of roughly 100 km s-1. The absorption lines toward 2128-123 are confined to only one or a few weak components and span a total velocity interval of less than almost-equal-to 20 km s-1. We present evidence that the lines of sight through the absorbers toward 0235 + 164 and 1229-021 penetrate material for which the velocity fields are dominated by systematic rather than by random motions and find that the line profiles are consistent with what is expected for rotating ensembles of clouds, These results suggest that for these two absorbers the absorption arises in individual clouds within single galaxy halos rather than, for example, in collections of coalescing subgalactic fragments, for which the expected velocity fields are more turbulent, or as a result of galaxy-galaxy clustering. Combining our data with similar data obtained for the z = 0.3950 absorber toward 1101-264 and considering the known impact parameters to the absorbing galaxies (determined from previous imaging studies), we tentatively conclude that lines of sight that impact absorbing regions at small impact parameters intercept many clouds, sample large velocity intervals, and are likely to show redshifted 21 cm absorption, whereas lines of sight that impact absorbing regions at large impact parameters intercept few clouds, sample small velocity intervals, and are unlikely to show redshifted 21 cm absorption. We interpret our data as favoring a model in which absorption arises in the extended halo regions of normal galaxies and suggest that such halos are dynamic entities that mark a significant phase of galaxy evolution.