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Astron. Astrophys. 357, 120-128 (2000)


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The head-tail structure of high-velocity clouds

A survey of the northern sky

C. Brüns 1,2, J. Kerp 1, P.M.W. Kalberla 1 and U. Mebold 1

1 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany (cbruens@astro.uni-bonn.de)
2 The Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping NSW 1710, Australia

Received 25 October 1999 / Accepted 8 February 2000

Abstract

We present new observational results on high-velocity clouds (HVCs) based on an analysis of the Leiden/Dwingeloo HI   survey. We cataloged all HVCs with [FORMULA]1019 cm-2 and found 252 clouds that form a representative flux limited sample. The detailed analysis of each individual HVC in this sample revealed a significant number of HVCs (nearly 20%) having simultaneously a velocity and a column density gradient. These HVCs have a cometary appearance in the position-velocity representation and are called henceforward head-tail HVCs (HT HVCs). The head is the region with the highest column density of the HVC, while the column density of the tail is in general much lower (by a factor of 2-4). The absolute majority of the cataloged HVCs belongs to the well known HVC complexes. With exception of the very faint HVC complex L, all HVC complexes contain HT HVCs. The HT HVCs were analyzed statistically with respect to their physical parameters like position, velocity ([FORMULA]), and column density. We found a linear correlation between the fraction of HVCs having a head-tail structure and the peak column density of the HVCs. While there is no correlation between the fraction of HT HVCs and [FORMULA], we found a dependence of the fraction of HT HVCs and [FORMULA]. There is no significant correlation between the fraction of HT HVCs and the parameters galactic longitude and latitude. The HT HVCs may be interpreted as HVCs that are currently interacting with their ambient medium. In the context of this model the tails represent material that is stripped off from the HVC core. We discuss the implications of this model for galactic and extragalactic HVCs.

Key words: ISM: clouds – ISM: structure – Galaxy: halo – Galaxy: structure

Send offprint requests to: C. Brüns, Bonn address

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000

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