Detection of a multi-phase ISM at z = 0.2212

Abstract

We present sensitive Giant Metrewave Radio Telescope (GMRT) and high-resolution Arecibo HI 21-cm observations of the damped Lyman-^α absorber (DLA) at z = 0:2212 towards OI 363 (B2 0738+313). The GMRT and Arecibo spectra are in excellent agreement and yield a spin temperature T_s = 890 ± 160 K, consistent with earlier lower sensitivity observations of the system. This value of T_s is far higher than spin temperatures measured for the Milky Way and local spirals but is similar to T_s values obtained in the majority of damped absorbers (T_s ≳ 1000 K). The high velocity resolution of the Arecibo spectra enables us to obtain estimates of physical conditions in the absorbing clouds by fitting multiple Gaussians to the absorption profile. The spectra are well fit by a three-component model with two narrow and one wide components, with temperatures T_k1 = 308 ± 24 K, T_k2 = 180 ± 30 K and T_k3 = 7600 ± 1250 K, respectively. The last of these is in excellent agreement with the expected temperatures for the WNM (5000-8000 K). Further, the mere fact that components are seen with lower temperatures than the estimated T_s implies that the absorber must have a multi-phase medium. We use the measured 21-cm optical depth and the above estimates of the kinetic temperature to obtain the HI column density in the various components. The total column density in the narrow components is found to be N_HI(CNM) ≤ 1:9±0:25ù10²⁰ cm ⁻², while that in the wide component is N_HI(WNM) ≥ 1:26±0:49ù10²¹ cm ⁻². Thus, the WNM contains at least 75% of the total HI in the z = 0:2212 DLA, unlike our Galaxy, in which the CNM and WNM have equitable contributions. As conjectured earlier (Chengalur & Kanekar 2000), this accounts for the difference in the spin temperatures of the z = 0:2212 system and local spirals, suggesting that the DLA is probably a dwarf or LSB type galaxy; this is also in agreement with optical studies (Turnshek et al. 2001). Finally, the total column density in the DLA is found to be N_HI ~ 1:45 ± 0:49 × 10²¹ cm −2, which agrees within the errors with the value of N_HI = 7:9 ± 1:4 × 10²⁰ cm⁻², obtained from the Lyman-α profile (Rao & Turnshek 1998). This reinforces our identification of the wide and narrow components as the WNM and CNM respectively.