On the statistics of galactic H I clouds

Abstract

The number density and random motions of interstellar H I clouds have been studied using an entirely novel method involving the comparison of the terminal velocities of H I absorption spectra in the direction of H II regions with their recombination line velocities. The conclusions of an earlier study attributing these velocity differences mainly to the random motions of the clouds were confirmed. It is also shown that these motions are better described in terms of two populations with quite different velocity dispersions than by a single population with an exponential distribution of velocities. The present analysis yields estimates of the number densities and dispersions of 7.7 + or - 1.8 per kpc and 4.1 + or - 0.7 km/s for the 'slow' clouds, and 0.6 + or - 0.15 per kpc and 15.2 + or - 3.2 km/s for the 'fast' clouds. These values are compared with earlier estimates and the implications discussed. It is concluded that the values for the slow clouds are an improvement over earlier determinations, while those for the population of fast clouds are underestimates because of selection effects in the data used.