Ionized gas in damped Ly alpha protogalaxies. I. Model-independent inferences from kinematic data

We investigate the kinematics of ionized and neutral gas in a sample of 35 damped Ly alpha systems (protogalaxies) using accurate quasar spectra obtai ned with HIRES, the echelle spectrograph on the Keck I 10 m telescope. Velo city profiles with resolution of similar to8 km s(-1) are obtained for high ions such as C IV and Si IV, and for intermediate ions such as Al III. Com bining these profiles with similar quality profiles obtained previously for low ions such as Fe II, we investigate the kinematic state of damped Ly al pha protogalaxies in the redshift range 1.8 < z < 4.4 by comparisons betwee n data for various ion pairs. We find that the damped Ly alpha protogalaxie s comprise distinct kinematic subsystems: a low-ion subsystem in which the low ions are physically associated with intermediate ions, and a high-ion s ubsystem containing neither low nor intermediate ions. The evidence for two subsystems stems from (1) differences between the widths of the velocity p rofiles, (2) misalignment in velocity space between the narrow components t hat make up the profiles in each subsystem, and (3) significant dissimilari ties between the mean velocities of the high-ion and low-ion velocity profi les. In every case we find that test statistics such as velocity width and various asymmetry parameters distribute differently for low and high ions. We also find that the absence of intermediate and low ions from the high-io n subsystem indicates the latter is optically thin at the Lyman limit. Desp ite misalignment between their velocity components, the low- and high-ion k inematic subsystems are interrelated. This is indicated by detection of a s tatistically significant C IV versus low-ion cross-correlation function. It is also indicated by a systematic effect in which the C IV velocity widths are greater than or equal to the low-ion velocity widths in 29 out of 32 s ystems. These phenomena are consistent with the location of the two subsyst ems in the same potential well.