Detection of Warm and Cold Phases of the Neutral ISM in a Damped Lyα Absorber

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Abstract
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 = 775 ± 100 K, the thermal kinetic temperatures of the two narrow components, calculated from their widths, are much lower: Tk ≤ 297 ± 3 and ≤103 ± 10 K, respectively. This is the first case of a redshifted absorption system for which Tk is measured to be less than . 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 Tk ≤ 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.
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