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-$\alpha$ absorber (DLA) at $z=0.2212$ towards OI 363. The spectra are in excellent agreement and yield a spin temperature $T_s=890\pm 160$ K, far higher than $T_s$ values measured in local spirals, but similar to values obtained in the majority of DLAs ($T_s \ga 1000$ 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 $T_{k_1}=308\pm 24$ K, $T_{k_2}=180\pm 30$ K and $T_{k_3}=7600\pm 1250$ K, respectively. The last of these is in excellent agreement with the expected temperatures for the WNM ($5000-8000$ K). 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 $\NHI (CNM) \le 1.9 \pm 0.25 \times 10^{20}$ cm, while that in the wide component is $\NHI (WNM) \ge 1.26 \pm 0.49 \times 10^{21}$ cm. 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; 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 (abridged).

8 pages, 2 figures. Accepted for publication in Astronomy and Astrophysics. Minor typos removed

References in corpus (3)

• Implications of 21cm observations for damped Ly-$\alpha$ systems
• HI 21cm absorption in low z damped lyman-alpha systems
• Lost and Found: The Damped Lyman Alpha Absorbers in the QSO OI 363

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