High‐Velocity Clouds as a Probe of the Global Structure of the Gaseous Galactic Halo

Abstract

We construct a simplified pressure equilibrium model for the confinement of high-velocity clouds (HVCs) in the Galactic halo of the Milky Way. The ambient pressure is obtained from a model for the distribution of the coronal gas density. This is assumed as an isothermal plasma in hydrostatic equilibrium with the gravitational field of the Galaxy. The cloud internal pressure, for either subsonic or supersonic HVCs, is expressed in terms of its observed parameters and as a function of distance from the Sun. The distances to three HVCs in complex M, observationally determined by Danly and coworkers in 1993, are used to set the values of the two free parameters of the model, namely, the thermal pressure of the coronal gas in the solar vicinity (~4 ? 10-13 dyn cm-2) and its temperature Tg ~ 106 K. This model can be more rigorously tested when a set of cloud distances, velocities, velocity dispersion, column densities, and angular sizes becomes available.