Ionized Gas in Damped Lyman Alpha Protogalaxies: I. Model-Independent Inferences From Kinematic Data

We investigate the kinematics of ionized and neutral gas in a sample of 35 damped Lyman alpha protogalaxies (DLAs) with HIRES on the Keck I 10 m telescope. Velocity profiles with resolution of ~ 8 km/s are obtained for high ions such as C IV and Si IV, and for intermediate ions such as Al III. Combining these profiles with accurate low-ion (e.g., Fe II) profiles, we investigate the kinematic state of DLAs in the redshift range 1.8 < z < 4.4 by comparisons between data for various ion pairs. We find the DLAs comprise distinct kinematic subsystems: a low ion subsystem in which the low ions are physically associated with intermediate ions, and a high subsystem containing neither low nor intermediate ions. The evidence for two subsystems stems from (a) differences between the widths of the velocity profiles, (b) misalignment in velocity space between the narrow components comprising the profiles in each subsystem, and (c) significant dissimilarities between the mean velocities of the high ion and low ion velocity profiles. In every case we find that test statistics such as velocity width and various asymmetry parameters distribute differently for low ions than for high ions. Despite misalignment in velocity space, the low and high ion kinematic subsystems are interrelated. This is indicated by detection of a statistically significant C IV versus low-ion cross correlation function, and by a systematic effect where the C IV velocity widths are greater than or equal to the low ion velocity widths in 29 out of 32 systems. These phenomena are consistent with the location of the two subsystems in the same potential well.