Compact high-velocity clouds around the Galaxy and M31

Abstract

We have observed 11 compact high-velocity clouds in the 21-cm line of neutral hydrogen with the 100-m telescope in Effelsberg. Our observations show that most of the clouds have a rather complex morphology. Head-tail structures and bow-shock shapes suggest that many of the CHVCs are interacting with an ambient medium, providing additional evidence for a circumgalactic CHVC population. In order to verify the concept of a circumgalactic population directly, we have started an HI blind survey to search for CHVCs in the neighbourhood of M31, using the Effelsberg telescope. We can confirm some of the extended HI structures discovered by Thilker et al. (2004) with the Green Bank Telescope (GBT), whereas we have not yet found any cloud similar to the CHVCs observed near the Milky Way. Our non-detection suggests an upper limit for the distance of CHVCs from their host galaxies of about 60 kpc.