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Astron. Astrophys. 356, L49-L52 (2000)

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Letter to the Editor

The vertical extent and kinematics of the HI in NGC 2403

W.E. Schaap 1, R. Sancisi 2,1 and R.A. Swaters 3

1 University of Groningen, Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
2 Osservatorio Astronomico, Bologna, Italy
3 Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, USA

Received 24 February 2000 / Accepted 8 March 2000


The neutral hydrogen line profiles along the major axis of the nearby spiral galaxy NGC 2403 show a wing towards the systemic velocity. This asymmetry can be explained with the presence of an abnormally thick HI disk (FWHM [FORMULA] 5 kpc) or with a two-component structure: a thin disk and a slowly rotating, thicker (1-3 kpc) HI layer. The latter model gives a better representation of the observations. These results throw a new light on the disk-halo connection in spiral galaxies. In particular, the decrease of rotational velocity with height above the plane may be the result of a galactic fountain flow. A vertically extended, slowly rotating HI layer may be common among spiral galaxies with high levels of star formation.

Key words: galaxies: individual: – galaxies: kinematics and dynamics – galaxies: structure

Send offprint requests to: W.E. Schaap

Correspondence to: wschaap@astro.rug.nl

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000