Anomalous H I in spiral galaxies

We present recent H I observations of the spiral galaxy NGC 2403, obtained with the VLA, which show a component of gas moving at anomalous velocity with respect to the regular rotation. This 'anomalous' gas is thought to form a thick disk or halo component surrounding the thin 'cold' disk. It has a mean rotation velocity 25-W km s(-1) lower than that of the disk and a 10-20 km s(-1) inflow towards the centre of the galaxy. A small fraction of this anomalous gas, concentrated in the central region of the galaxy, has projected velocity differences from the rotation curve of up to 150 km s(-1). This Hi study of NGC 2403 has been followed up with Chandra observations which show a diffuse, hot X-ray emitting gas component. These results point at an active disk-halo connection and at a galactic fountain type of phenomenon.