On the statistics of Galactic HI clouds

Abstract

The number density and random motions of interstellar HI clouds have been studied using an entirely novel method involving the comparison of the terminal velocities of HI absorption spectra in the direction of HI1 regions with their recombination line velocities. We confirm the conclusions of an earlier study attributing these velocity differences mainly to the random motions of the clouds. We also show that these motions are better described in terms of two populations wit11 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 ns = 7.7 rl: 1.8 ltpc-I as = 4.1 + 0.7 kms-I for the "slow" clouds, and nf =0.6 + 0.1 5 kpc' , af = 15.2 + 3.2 kms-' for the "f6t" 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, but that those for the population of fast clouds are underestimates because of selection effects in the data used.