Deep H I Survey of the Spiral Galaxy NGC 2403
  High sensitivity H I observations of the nearby spiral galaxy NGC 2403 obtained with the VLA are presented and discussed. The properties of the extended, differentially rotating H I layer with its H I holes, spiral structure and outer warp are described. In addition, these new data reveal the presence of a faint, extended and kinematically anomalous component. This shows up in the H I line profiles as extended wings of emission towards the systemic velocity. In the central regions these wings are very broad (up to 150 km/s) and indicate large deviations from circular motion. We have separated the anomalous gas component from the cold disk and have obtained for it a separate velocity field and a separate rotation curve. The mass of the anomalous component is 1/10 of the total H I mass. The rotation velocity of the anomalous gas is 25-50 km/s lower than that of the disk. Its velocity field has non-orthogonal major and minor axes that we interpret as due to an overall inflow motion of 10-20 km/s towards the centre of the galaxy. The picture emerging from these observations is that of a cold H I disk surrounded by a thick and clumpy H I layer characterized by slower rotation and inflow motion towards the center. The origin of this anomalous gas layer is unclear. It is likely, however, that it is related to the high rate of star formation in the disk of NGC 2403 and that its kinematics is the result of a galactic fountain type of mechanism. We suggest that these anomalous H I complexes may be analogous to a part of the High Velocity Clouds of our Galaxy. 
Filippo Fraternali (1 and 2), Gustaaf van Moorsel (3), Renzo Sancisi   (2 and 4), Tom Oosterloo (5) ((1) IRA, Bologna, I, (2...