High Velocity Gas in the Halos of Spiral Galaxies

Recent, high sensitivity, HI observations of nearby spiral galaxies show that their thin `cold' disks are surrounded by thick layers (halos) of neutral gas with anomalous kinematics. We present results for three galaxies viewed at different inclination angles: NGC891 (edge-on), NGC2403 (i=60 deg), and NGC6946 (almost face-on). These studies show the presence of halo gas up to distances of 10-15 kpc from the plane. Such gas has a mean rotation 25-50 km/s lower than that of the gas in the plane, and some complexes are detected at very high velocities, up to 200-300 km/s. The nature and origin of this halo gas are poorly understood. It can either be the result of a galactic fountain or of accretion from the intergalactic medium. It is probably the analogous of some of the High Velocity Clouds (HVCs) of the Milky Way.