H I spin temperatures and heating requirements in outer regions of disk galaxies

We show how to use 21 cm emission and absorption studies to estimate the heat inputs to the neutral gas in low pressure environments, such as in outer disks of spiral galaxies, in galactic halos or in intergalactic space. For a range of model parameters we calculate the gas kinetic and spin temperatures (T K and T S ) and the relation between T S and the heat input to the gas. We outline the conditions for a two-phase medium to exist. We find that although T S can be much smaller than T K , T S is always 3 K for column densities greater than 5×10 18 cm −2