Extra-planar H I in the starburst galaxy NGC 253

Observations of the nearby starburst galaxy NGC 253 in the 21-cm line reveal the presence of neutral hydrogen in the halo, up to 12 kpc from the galactic plane. This extra-planar H I is found in only one half of the galaxy and is concentrated in a half-ring structure and plumes which are lagging in rotation with respect to the disk. The H I plumes are seen bordering the bright Halpha and X-ray halo emission. It is likely that, as proposed earlier for the Halpha and the X-rays, the origin of the extra-planar H I is also related to the central starburst and to the active star formation in the disk. A minor merger and gas accretion are also discussed as possible explanations.

The H I disk is less extended than the stellar disk. This may be the result of ionization of its outer parts or, alternatively, of tidal or ram pressure stripping.