Galaxy Formation and the Kinematics of Damped Lyα Systems

A model of damped lyman alpha systems is presented based on randomly movingclouds in spherical halos. We use the Press-Schechter model for the abundanceof halos, and assume that each halo has a similar population of clouds, withtotal mass and spatial distribution constrained to fit observations of thecolumn density distribution. We show that the kinematics of the multipleabsorbing components revealed in absorption profiles of the low-ionizationlines, presented by Prochaska & Wolfe, are consistent with our spherical halomodel. The presence of multiple absorbing components with a large covering factor,combined with the small impact parameters of the systems predicted in ouranalytical model and in numerical simulations, implies a high rate of energydissipation in cloud collisions. We calculate the rate of energy dissipation inour model, and show that it is far greater than the rate at which energy can besupplied by gravitational mergers of halos. This poses a problem for the modelof merging protogalactic clumps of Haehnelt et al., based on numericalsimulations. We also present new constraints on the amplitude of the powerspectrum in hierarchical theories required to account for the observed velocitydispersion in the absorbers. We find that the linearly extrapolated rmsfluctuation on spheres of radius HR = 100 km/s at z=4 must be greater than0.75. Although this limit is obtained only for our specific model of theabsorbing components, it should not be highly model-dependent because thevelocity dispersion of the absorbers is essentially determined by the velocitydispersion of the halos where the gas is moving.Comment: submitted to ApJ, 25 pages, 14 figure