Résumé
MNRAS 296 (1998) 430 We employ observationally determined intrinsic velocity widths and column
densities of damped Lyman-alpha systems at high redshift to investigate the
distribution of baryons in protogalaxies within the context of a standard cold
dark matter model. We proceed under the assumption that damped Lyman alpha
systems represent a population of cold, rotationally supported, protogalactic
disks and that the abundance of protogalactic halos is well approximated by a
cold dark matter model with critical density and vanishing cosmological
constant. Using conditional cross sections to observe a damped system with a
given velocity width and column density, we compare observationally inferred
velocity width and column density distributions to the corresponding
theoretically determined distributions for a variety of disk parameters and CDM
normalizations. In general, we find that the observations can not be reproduced
by the models for most disk parameters and CDM normalizations. Whereas the
column density distribution favors small disks with large neutral gas fraction,
the velocity width distribution favors large and thick disks with small neutral
gas fraction. The possible resolutions of this problem in the context of this
CDM model may be: (1) an increased contribution of rapidly rotating disks
within massive dark matter halos to damped Lyman-alpha absorption or (2) the
abandoning of simple disk models within this CDM model for damped Lyman-alpha
systems at high redshift. Here the first possibility may be achieved by
supposing that damped Lya system formation only occurs in halos with fairly
large circular velocities and the second possibility may result from a large
contribution of mergers and double-disks to damped Lya absorption at high
redshift.