Effelsberg H I observations of compact high-velocity clouds

Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: twestmei@astro.uni-bonn.de

Received: 6 May 2004
Accepted: 24 November 2004

Abstract

We have mapped 11 compact high-velocity clouds (CHVCs) in the 21-cm line emission of neutral, atomic hydrogen, using the Effelsberg 100-m radio telescope. The aim of our observations was to study the overall distribution of the warm neutral medium of CHVCs with high sensitivity. The achieved baseline rms of at the original velocity resolution allows us to search for evidence of ram-pressure interaction with the ambient medium. In addition, we have obtained spectra along an appropriate axis across each CHVC with longer integration times and denser angular sampling. These deep slices with allow us to determine the column density profile in greater detail as well as the velocity and line width gradient across each cloud.
The most outstanding result of our observations is the complexity of the  column density distribution and the line profiles of the investigated CHVCs. We have found only one cloud with a spherically-symmetric appearance. Among the remaining clouds we observe head-tail structures, bow-shock shapes, and objects with irregular shapes. These complex morphologies in combination with the obtained physical parameters suggest that ram-pressure interactions with an ambient medium may play a significant role in shaping some of the CHVCs from our sample. These results are consistent with a circumgalactic distribution of CHVCs with typical distances of the order of 100 kpc. The pressure of the ambient medium might also stabilise CHVCs in addition to their own gravitational potential.