2005A&A...436..101W

We have undertaken high-resolution follow-up of a sample of high velocity HI clouds apparently associated with M 31. Our sample was chosen from the population of high-velocity clouds (HVCs) detected out to 50 kpc projected radius of the Andromeda Galaxy by Thilker et al. (2004ApJ...601L..39T) with the Green Bank Telescope. Nine pointings were observed with the Westerbork Synthesis Radio Telescope to determine the physical parameters of these objects and to find clues to their origin. One additional pointing was directed at a similar object near M 33. At 2' resolution we detect 16 individual HVCs around M 31 and 1 HVC near M 33 with typical HI masses of a few times 10⁵M_☉ and sizes of the order of 1kpc. Estimates of the dynamical and virial masses of some of the HVCs indicate that they are likely gravitationally dominated by additional mass components such as dark matter or ionised gas. Twelve of the clouds are concentrated in an area of only 1°x1° at a projected separation of less than 15 kpc from the disk of M 31. This HVC complex has a rather complicated morphological and kinematical structure and partly overlaps with the giant stellar stream of M 31, suggesting a tidal origin. Another detected feature is in close proximity, in both position and velocity, with NGC 205, perhaps also indicative of tidal processes. Other HVCs in our survey are isolated and might represent primordial, dark-matter dominated clouds.