Extra-planar HI 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 HI is found only in 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 HI 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, also the origin of the extra-planar HI is 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 HI 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. 
Rense Boomsma (1), Tom Oosterloo (2), Filippo Fraternali (3 and 2),   Thijs van der Hulst (1), Renzo Sancisi (4 and 1) ((1) ...