Relics of structure formation: extra-planar gas and high-velocity clouds around the Andromeda Galaxy

Using the 100-m radio telescope at Effelsberg, we mapped a large area around the Andromeda Galaxy in the 21 cm line emission of neutral hydrogen to search for high-velocity clouds (HVCs) out to large projected distances in excess of 100 kpc. Our 3 H I mass sensitivity for the warm neutral medium is 8 10 4 M . We can confirm the existence of a population of HVCs near M31 with typical H I masses of a few times 10 5 M . However, we did not detect any HVCs beyond a projected distance of about 50 kpc from M31, suggesting that HVCs are generally found in proximity of large spiral galaxies at typical distances of a few 10 kpc.