The Relics of Structure Formation: Extra-Planar Gas and High-Velocity Clouds Around M31

We mapped a large region around M31 in H I with the 100-m radio telescope at Effelsberg to search for high-velocity clouds (HVCs) out to large projected distances in excess of 100 kpc. We argue that tidal stripping may have played an important role in forming several of the 17 detected HVCs, whereas other clouds could be primordial dark-matter satellites.