Ionized Gas in Damped Lyα Protogalaxies. I. Model‐independent Inferences from Kinematic Data

Abstract

We investigate the kinematics of ionized and neutral gas in a sample of 35 damped Lyα systems (protogalaxies) using accurate quasar spectra obtained with HIRES, the echelle spectrograph on the Keck I 10 m telescope. Velocity profiles with resolution of ~8 km s-1 are obtained for high ions such as C IV and Si IV, and for intermediate ions such as Al III. Combining these profiles with similar quality profiles obtained previously for low ions such as Fe II, we investigate the kinematic state of damped Lyα protogalaxies in the redshift range 1.8 < z < 4.4 by comparisons between data for various ion pairs. We find that the damped Lyα protogalaxies comprise distinct kinematic subsystems: a low-ion subsystem in which the low ions are physically associated with intermediate ions, and a high-ion subsystem containing neither low nor intermediate ions. The evidence for two subsystems stems from (1) differences between the widths of the velocity profiles, (2) misalignment in velocity space between the narrow components that make up the profiles in each subsystem, and (3) 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 and high ions. We also find that the absence of intermediate and low ions from the high-ion subsystem indicates the latter is optically thin at the Lyman limit. Despite misalignment between their velocity components, 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. 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 systems. These phenomena are consistent with the location of the two subsystems in the same potential well.