HVCs probing a gaseous Galactic halo

Abstract

We study the hypothesis that high-velocity clouds (HVCs) may originate from instabilities within the gaseous phase of the Galactic halo. According to the hydrostatic equilibrium model of Kalberla & Kerp (1998), we determine the probability distribution of occurrence of instabilities within the Galactic halo. Instabilities may produce condensations within the Galactic halo beyond a z-distances of 4 kpc, which are accelerated by gravity and decelerated by drag-forces. We present evidence that the HI high-velocity dispersion component, discovered by Kalberla et al. (1998) is associated with high-velocity-halo gas. The physical properties of this high-velocity-halo gas are similar to the recently detected highly-ionized HVCs by Sembach et al. (1995, 1998). Accordingly, the HI high-velocity dispersion component may be the neutral tracer of the turbulent gas motions within the Galactic halo. Our calculations demonstrate, that the sky-averaged signal of in-falling condensations does not differ significantly from the HI emission of the turbulent Galactic halo gas.