The vertical structure and kinematics of HI in spiral galaxies

We discuss the distribution and kinematics of neutral hydrogen in the halos of spiral galaxies. We focus in particular on new results obtained for the nearby Sc galaxy NGC 2403 which have revealed the presence of extended HI emission (here referred to as the beard') at anomalous velocities with respect to the cold’ HI disk. Modeling shows that this component has a mass of about 1/10 of the total HI mass and is probably located in the halo region. Its kinematics differs from that of the thin HI disk: it rotates more slowly and shows radial inflow. The origin of this anomalous gas component is unknown. It could be the result of a galactic fountain or of accretion of extragalactic `primordial’ gas.