2000A&A...356L..49S

The neutral hydrogen line profiles along the major axis of the nearby spiral galaxy NGC 2403 show a wing towards the systemic velocity. This asymmetry can be explained with the presence of an abnormally thick HI disk (FWHM∼5kpc) or with a two-component structure: a thin disk and a slowly rotating, thicker (1-3kpc) HI layer. The latter model gives a better representation of the observations. These results throw a new light on the disk-halo connection in spiral galaxies. In particular, the decrease of rotational velocity with height above the plane may be the result of a galactic fountain flow. A vertically extended, slowly rotating HI layer may be common among spiral galaxies with high levels of star formation.