The relics of galaxy evolution: High-velocity clouds around the Andromeda Galaxy

High-velocity clouds (HVCs) are defined by their high radial velocities which are incompatible with a participation in the normal rotation of our Galaxy. They are observed mainly in the 21 cm line of neutral hydrogen. HVCs can be found all over the sky in the form of large complexes as well as compact, isolated clouds. To substantiate the concept of a circum-galactic population of HVCs we carried out an HI blind survey of a large area around the Andromeda Galaxy (M31) with the 100 m radio telescope at Effelsberg. We detected several HVCs near the disk of M31, but not a single HVC was found beyond a projected distance of about 50 kpc, indicating that HVCs are located in the vicinity of the large galaxies. Follow-up synthesis observations with the Westerbork synthesis radio telescope show that the detected HVCs have diameters of the order of 1 kpc and HI masses of typically a few times 105M⊙. Some of the HVCs near M31 are likely associated with the recently discovered giant stellar stream. Another HVC near NGC 205 may also be of tidal origin, whereas other HVCs around M31 are isolated and may be considered candidates for the primordial, gaseous dark matter satellites predicted by CDM structure formation scenarios.