Ionized gas in damped Ly alpha protogalaxies. II. Comparison between models and the kinematic data

We test semianalytic models for galaxy formation with accurate kinematic da ta of damped Ly alpha proto-galaxies presented in a companion paper. The mo dels envisage centrifugally supported exponential disks at the centers of d ark matter halos, which are filled with ionized gas undergoing radial infal l to the disks. The halo masses are drawn from cross section weighted mass distributions predicted by cold dark matter (CDM) cosmogonies, or by the nu ll hypothesis that the dark matter mass distribution has not evolved since z similar to 3 (i.e., the Tully-Fisher [TF] models). In our models, C IV ab sorption lines detected in damped Lya protogalaxies arise in infalling ioni zed clouds, while the low-ion absorption lines arise from neutral gas in th e disks. Using Monte Carlo methods, we find that : (1) The CDM models are i ncompatible with the low-ion statistics at more than 99% confidence, wherea s some TF models cannot be ruled out at more than 88% confidence. (2) Both CDM and TF models are in general agreement with the observed distribution o f C IV velocity widths. (3) The CDM models generate differences between the mean velocities of C IV and low-ion profiles that are compatible with the data, while the TF model produces differences in the means that are too lar ge. (4) Both CDM and TF models produce ratios of C IV to low-ion velocity w idths that are too large. (5) Neither CDM nor TF models generate C IV versu s low-ion cross-correlation functions compatible with the data. While it is possible to select model parameters resulting in agreement between the mod els and the data, the fundamental problem is that the disk-halo configurati on assumed in both cosmogonies does not produce significant overlap in velo city space between C IV and low-ion velocity profiles. We conjecture that i ncluding the angular momentum of the infalling clouds will increase the ove rlap between C IV and low-ion profiles.