Cold neutral hydrogen gas in galaxies
This review summarizes recent studies of the cold neutral hydrogen gas associated with galaxies probed via the HI 21-cm absorption line. HI 21-cm absorption against background radio-loud quasars is a powerful tool to study the neutral gas distribution and kinematics in foreground galaxies from kilo-parsec toparsec scales. At low redshifts ($z$ < 0.4), it has been used to characterize the distribution of high column density neutral gas around galaxies and study the connection of this gas with the galaxy’s optical properties. The neutral gas around galaxies has been found to be patchy in distribution, with variations in optical depth observed at both kilo-parsec and parsec scales. At high redshifts ($z$ > 0.5), HI 21-cm absorption has been used to study the neutral gas in metal or Lyman − $\alpha$ absorption-selected galaxies. It has been found to be closely linked with the metal and dust content of the gas. Trends of various properties like incidence, spin temperature and velocity width of HI 21-cm absorption with redshift have been studied, which imply evolution of coldgas properties in galaxies with cosmic time. Upcoming large blind surveys of HI 21-cm absorption with next generation radio telescopes are expected to determine accurately the redshift evolution of the number densityof HI 21-cm absorbers per unit redshift and hence understand what drives the global star formation rate density evolution.
Volume 41, 2020
Continuous Article Publishing mode
Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.
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