A New, Kinematically Anomalous H I Component in the Spiral Galaxy NGC 2403

We discuss new, high-sensitivity H I observations of the spiral galaxy NGC 2403, which show extended emission at anomalous velocities with respect to the "cold" disk. This "anomalous" gas component (~1/10 of the total H I mass) is probably located in the region of the halo and rotates more slowly (~20-50 km s-1) than the gas in the disk. Moreover, it shows a distortion in the velocity field that we interpret as a large-scale radial motion (10-20 km s-1 inflow) toward the center of the galaxy. The most likely explanation for its origin and kinematics seems to be that of a galactic fountain. There is, however, a significant part of the anomalous gas that seems to be moving contrary to rotation and is difficult to understand in such a picture. These anomalous gas complexes discovered in NGC 2403 may be analogous to the high-velocity clouds of our Galaxy. They may be rather common in spiral galaxies and not have been detected yet for lack of sensitivity.