Thermal-convective instability of interstellar gas and the origin of high-velocity HI clouds

Abstract

A mechanism is proposed for the formation of high velocity HI clouds falling onto the plane of the galaxy. As a spiral shock wave passes through the interstellar gas, conditions arise under which gas can go over into a two-phase state (thermal instability). In an external gravitational field (in the given case, in the field of the disk of the galaxy) the thermal instability leads to convection -- the colder gas sinks. During the nonlinear stage the cold clouds become separated and continue to fall toward the plane of the galaxy. The cloud velocities reach the observed values of approximately 70 km/sec.