Kinematics of diffuse interstellar clouds: recent GMRT results

Abstract

A high latitude HI 21 cm-line absorption survey towards extragalactic sources was recently completed using the Giant Meterwave Radio Telescope (GMRT). A total of 104 sources with |b|>15o and with a 21 cm ¤ux density greater than 1 Jy were observed for ~120 hours. With an optical depth detection limit of ~ 0.01 this is the most sensitive high-latitude survey as yet. Most of the detected HI 21 cm-line absorption features belong to a population with a velocity dispersion of ~7.6 km s-1. These are the standard HI clouds and have been well-studied for decades. However, we also detect a second population of absorbing clouds with a velocity dispersion of ~ 21 km s-1. About 20% of the total population of absorbing clouds belong to this population. This new population of fast clouds can be identified with a similar velocity dispersion Ca II absorbing clouds and with the Halo clouds recently detected in HI emission from Green Bank.