Extra-planar H I in the starburst galaxy NGC 253

¹ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: boomsma@astro.rug.nl
² ASTRON, Dwingeloo, The Netherlands
³ Theoretical Physics, University of Oxford, UK
⁴ INAF - Osservatorio Astronomico di Bologna, Bologna, Italy

Received: 22 July 2004
Accepted: 23 September 2004

Abstract

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 Hα and X-ray halo emission. It is likely that, as proposed earlier for the Hα 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.