Galaxy formation and the kinematics of damped Lyα systems

A model of damped Ly alpha systems is presented based on randomly moving clouds in spherical halos. We use the Press-Schechter model for the abundance of halos and assume that each halo has a similar population of clouds, with total mass and spatial distribution constrained to fit observations of the column density distribution. We show that the kinematics of the multiple absorbing components revealed in absorption profiles of the low-ionization lines, presented by Prochaska & Wolfe, are consistent with our spherical halo model. The presence of multiple absorbing components with a large covering factor, combined with the small impact parameters of the systems predicted in our analytical model and in numerical simulations, implies a high rate of energy dissipation in cloud collisions. We calculate the rate of energy dissipation in our model and show that it is far greater than the rate at which energy can be supplied by gravitational mergers of halos. This poses a possible problem for the model of merging protogalactic clumps of Haehnelt et al., based on numerical simulations. We also present new constraints on the amplitude of the power spectrum in hierarchical theories required to account for the observed velocity dispersion in the absorbers. We find that the linearly extrapolated mts fluctuation at redshift z = 4 On spheres of radius R =100 km s(-1) H-1 (z) [where H(z) is the Hubble constant at redshift z] must be greater than 0.75. Although this limit is obtained only for our specific model of the absorbing components, it should not be highly model-dependent because the velocity dispersion of the absorbers is essentially determined by the velocity dispersion of the halos where the gas is moving.