Protogalactic Disk Models of Damped Lyα Kinematics

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Abstract
We present new observational results on the kinematics of the damped Lyα systems. Our full sample now comprises 31 low-ion profiles and exhibits characteristics similar to those of the sample from our previous paper.The primary exception is that the new distribution of velocity widths includes values out to a maximum of nearly 300 km s-1, ≈ 100 km s-1 greater than the previous maximum. These high velocity width systems will significantly leverage models introduced to explain the damped Lyα systems. Comparing the characteristics from low-redshift and high-redshift subsamples, we find no evidence for significant evolution in the kinematic properties of protogalaxies from z = 2.0 to z = 3.3. The new observations give greater statistical significance to the main conclusions of our first paper. In particular, those models inconsistent with the damped Lyα observations in the first paper are ruled out at even higher levels of confidence. At the same time, the observations are consistent with a population of rapidly rotating thick disks (the TRD model) at high redshift, as predicted by cosmologies with early structure formation. Buoyed by the success of the TRD model, we investigate it more closely by considering more realistic disk properties. Our goal is to demonstrate the statistical power of the damped Lyα observations by investigating the robustness of the TRD model. In particular, we study the effects of warping, realistic rotation curves, and photoionization on the kinematics of disks in the TRD model. The principal results are the following: (1) Disk warping has only a minimal effect on the kinematic results, primarily influencing the effective disk thickness. (2) The TRD model is robust to more realistic rotation curves; we point out, however, that the rotation curve derived from centrifugal equilibrium with H I gas alone does not yield acceptable results but that rather flat rotation curves such as those generated by dark matter halos are required. (3) The effects of photoionization require thicker disks to give consistent velocity width distributions.
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