The research will involve a study of the cool atomic phase of the interstellar medium in the disks of nearby spiral galaxies. The purpose of the research is to measure the relative fractions of warm and cool interstellar atomic hydrogen (HI). This fraction is a sensitive function of the heating process and the interstellar pressure. The second objective is to measure the cloud size spectrum in the mass range one hundred to several thousand solar masses. The observations will show whether the abundance of these of these atomic clouds increases with decreasing galactic radius, as do the molecular clouds in most spirals, or whether the atomic clouds are depopulated where the molecular clouds are most abundant. The third objective is to measure the cross-sections of spiral galaxies for 21 cm wavelength absorption. This will provide a better understanding of the origin of HI absorption lines at high redshifts. The observing technique will be to measure 21 cm absorption spectra through the disks of 50 nearby spiral galaxies. All Shapley- Ames catalog of galaxies (blue magnitude less than 12) with suitable background sources will be observed, and very nearby galaxies (designated M31, M33, and M81) will be studied along several lines of sight. The recent expansion of the Very large Array (VLA) on-line data reduction system makes this feasible for the first time. The basic purpose of this research will be to understand the abundance and distribution of cool interstellar clouds in spiral galaxies and to determine whether molecular and atomic clouds are a single population or two distinct species.
