Modelling the HI halo of the Milky Way

Aims: We studied the global distribution and kinematics of the extra-planar neutral gas in the Milky Way. Methods: We built 3D models for a series of Galactic H I layers, projected them for an inside view, and compared them with the Leiden-Argentina-Bonn 21-cm observations. Results: We show that the Milky Way disk is surrounded by an extended halo of neutral gas with a vertical scale-height of 1.6+0.6-0.4 kpc and an H I mass of 3.2+1.0-0.9 × 108 M⊙, which is ~5-10% of the total Galactic H I. This H I halo rotates more slowly than the disk with a vertical velocity gradient of -15 ± 4 km s-1 kpc-1. We found evidence for a global infall motion in the halo, both vertical (20+5-7 km s-1) and radial (30+7-5 km s-1). Conclusions: The Milky Way H I extra-planar layer shows properties similar to the halos of external galaxies, which is compatible with it being predominantly produced by supernova explosions in the disk. It is most likely composed of distinct gas complexes with masses of ~104-5 M⊙, of which the intermediate velocity clouds are the local manifestations. The classical high-velocity clouds appear to be a separate population.