ar X iv : a st ro - p h / 99 11 14 2 v 1 9 N ov 1 99 9 Detection of Warm and Cold Phases of the Neutral ISM in a Damped Ly α Absorber

We present a detailed study of the H i 21cm absorption system at z = 0.0912 towards 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 ∆v 1 = 3.7 and ∆v 2 = 2.2 km s −1 (main and secondary component 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 21cm 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 21cm lines. The third component is interpreted as representing the warm phase gas with with T k ≤ 5050 ± 950 K. The combined column density of the three 21cm components is approximately equal to that derived from fits to the damped Lyα line.