The temperature of the neutral hydrogen in the galaxy

Abstract

We present high-velocity resolution HI 21-cm absorption studies along 12 sightlines toward compact background radio sources, using the upgraded Giant Metrewave Radio Telescope (uGMRT). Our observations achieve an optical depth root mean square (RMS) noise of approximately 10−6 per kms−1 velocity channel, enabling the detection of absorption features from thewarmneutral medium (WNM). HI 21-cm absorption is detected along 10 of the 12 sightlines; no absorption is found toward 3C345 and 0023-236. For each sightline, we present the absorption spectra, the corresponding emission spectra from the Leiden-Argentine-Bonn (LAB) survey along nearby directions, and the derived spin temperature (𝑇𝑆) spectra. In all cases, we detect maximum spin temperatures of 𝑇𝑆 ≥ 103 K, indicating that we are detecting the warm neutral medium.