Ionized Gas in Damped Lyα Protogalaxies. II. Comparison between Models and the Kinematic Data

Abstract

We test semianalytic models for galaxy formation with accurate kinematic data of damped Lyα protogalaxies presented in a companion paper. The models envisage centrifugally supported exponential disks at the centers of dark matter halos, which are filled with ionized gas undergoing radial infall to the disks. The halo masses are drawn from cross section weighted mass distributions predicted by cold dark matter (CDM) cosmogonies, or by the null hypothesis that the dark matter mass distribution has not evolved since z ~ 3 (i.e., the Tully-Fisher [TF] models). In our models, C IV absorption lines detected in damped Lyα protogalaxies arise in infalling ionized clouds, while the low-ion absorption lines arise from neutral gas in the disks. Using Monte Carlo methods, we find that: (1) The CDM models are incompatible with the low-ion statistics at more than 99% confidence, whereas 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 of 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 large. (4) Both CDM and TF models produce ratios of C IV to low-ion velocity widths that are too large. (5) Neither CDM nor TF models generate C IV versus low-ion cross-correlation functions compatible with the data. While it is possible to select model parameters resulting in agreement between the models and the data, the fundamental problem is that the disk-halo configuration assumed in both cosmogonies does not produce significant overlap in velocity space between C IV and low-ion velocity profiles. We conjecture that including the angular momentum of the infalling clouds will increase the overlap between C IV and low-ion profiles.