Cool HI Disks in Galaxies

Abstract

Because of the sensitivity and resolution, both spatial and spectral, at centimeter wavelengths and the large amounts of interstellar gas in the form of neutral atomic hydrogen (HI) in typical galaxies, the 21-cm HI line has served as a valuable tool in probing the disk component of galaxies, both structurally and dynamically. Even without spatial information, the global 21-cm profile provides three important parameters: the redshift, the total HI line flux, and line width, the latter a measure of the Doppler broadening. The use of the HI line for redshift surveys has been discussed by Giovanelli & Haynes (1990); HI redshifts remain among the most accurate determinations available to redshift surveys and have been measured for more than 12000 galaxies. The total HI line flux is used as a measure of the potential for future star formation and as an indicator of past gas removal. Spatial mapping of the HI distribution allows the intercomparison of the gas and stellar distributions. The global HI line width in rotationally dominated galaxies has been used via the Tully-Fisher relation (e.g. Rowan-Robinson 1985) as a predictor of galaxy luminosity, under the assumptions that rotation speed predicts mass and mass predicts luminosity. Mapping of the velocity field provides an indication of the distribution of mass, both luminous and dark.