2000ApJ...532..146L

We present a detailed study of the H I 21 cm absorption system at z=0.0912 toward the radio quasar B0738+313. The uncommonly narrow main absorption line and weak secondary line are resolved for the first time and have FWHM velocities of Δv₁=3.7 and Δv₂=2.2 km.s^–1 (main and secondary components, respectively). In addition, we find it necessary to add a third, broader shallow component to obtain a good fit to the spectrum. Although the harmonic mean spin temperature calculated by comparison of the 21 cm lines to the damped Lyα line is T_s=775±100 K, the thermal kinetic temperatures of the two narrow components, calculated from their widths, are much lower: T_k≤297±3 and ≤103±10 K, respectively. This is the first case of a redshifted absorption system for which T_k is measured to be less than T_s. We discuss this result in the context of a two-phase gas model, in which the damped Lyα gas is sensitive to a significant neutral column density of warm-phase gas as well as the cold-phase gas of the narrow 21 cm lines. The third component is interpreted as representing the warm-phase gas with T_k≤5050±950 K. The combined column density of the three 21 cm components is approximately equal to that derived from fits to the damped Lyα line.