The spashdown of high velocity clouds

Interstellar clouds falling towards the Galaxy appear to be slowing down as they approach the Galactic plane. If interstellar drag on these clouds is sufficiently strong, they will approach a terminal velocity \nu(T)(z)\ = root 2Ng(z)/C(D)n(h)(z), where the deceleration due to drag balances gravitational acceleration. Since the terminal velocity decreases with z, clouds should slow down as they fall. For a cloud of fixed column density, the terminal velocity is a function only of position in space and depends upon a model of gravity, g(z), and ambient gas density, n(h)(z). Therefore, by measuring a cloud velocity and column density, the cloud position can be predicted and tested against the actual cloud distance as bracketed by UV or optical absorption line studies. The agreement between predicted cloud distances and observed is found to be good. It is argued that cloud morphology, energy dissipation, and internal cloud pressures are also consistent with the terminal velocity hypothesis. For clouds above 1 kiloparsec, we predict that the internal pressure p proportional to N, and that the internal magnetic field, B proportional to root N. The terminal velocity hypothesis is significant because (1) it can be used as a distance indicator; (2) it can be used as a probe of the density of the inter-cloud medium; and (3) it provides a measure of how durable clouds are against hydrodynamic instabilities and other destruction mechanisms. It can thus provide new insights on physical process in the ISM.