The vertical extent and kinematics of the HI in NGC 2403

The neutral hydrogen line profiles along the major axis of the nearby spira l 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 si milar to 5 kpc) or with a two-component structure: a thin disk and a slowly rotating, thicker (1-3 kpc) HI layer. The latter model gives a better repr esentation of the observations. These results throw a new light on the disk -halo connection in spiral galaxies. In particular, the decrease of rotatio nal velocity with height above the plane may be the result of a galactic fo untain flow. A vertically extended, slowly rotating HI layer may be common among spiral galaxies with high levels of star formation.