HALO MODEL FOR INTERMEDIATE-REDSHIFT QUASAR ABSORPTION SYSTEMS

We have constructed a simple model for the halo regions of galaxies responsible for producing sharp metal line absorption systems in the spectra of quasars at z approximately 1.5. The model assumes spherical halo. The cloud number density as well as the metal abundance decrease with distance from the galactic center. We have constructed photoionization models for individual clouds to obtain the column densities of various ions in the clouds, for a range of values for ionization parameter, metallicity, and size of the clouds. The equivalent widths of a number of lines produced by individual clouds have been calculated and used to obtain the total equivalent width of these lines along a given line of sight through the halo. The predicted equivalent width distribution for Mg II is compared with observations in order to restrict the possible range of values for cloud parameters. For the range of parameters considered, the models can produce Lyman limit systems (LLSs) with and without lines of singly ionized species, LLSs without accompanying metal lines, highly ionized systems with tau(LLS) < 1, as well as damped Lyman alpha systems. Equivalent width distribution for C iv lines associated with Mg II lines as well as for C IV lines not associated with Mg II lines have been obtained from observations by performing maximum likelihood analysis and are shown to be consistent with the model results. The models reproduce the doublet ratios of Mg II and associated C IV lines as a function of their equivalent widths. The doublet ratios of C IV lines not associated with Mg II can not be reproduced satisfactorily by our model. The range of observed equivalent width ratios of several lines and the probability of these ratios exceeding 1 are consistent with the model results, except for the observed high values of Si IV/C IV, which cannot be reproduced by the models. Observed equivalent widths of different lines in a number of absorption systems are compared individually with model predictions. Barring a few exceptions, the observations are well reproduced by our models. The impact parameters are in the range 10-40 kpc, which is similar to the observed range. All these comparisons with observations show that a range in column densities, abundance, and ionization parameter is needed in order to explain the observations. The values of the ionization parameter close to 10(-2) are more likely than 10(-3). Neutral hydrogen column density distribution obtained, based on our models, seems to be consistent with the observed distribution for column densities between (5 x 10(16))-(2 x 10(21)) cm-2.