  A model of damped lyman 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 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 rms
fluctuation on spheres of radius HR = 100 km/s at z=4 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.
