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 3σ H i mass sensitivity for the warm neutral medium is 8 × 10⁴ M_⊙. We can confirm the existence of a population of HVCs near M31 with typical H i masses of a few times 10⁵ 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.

Comparison with cold dark matter (CDM)-based models and simulations suggests that only a few of the detected HVCs could be associated with primordial dark matter satellites, whereas others are most likely the result of tidal stripping. The lack of clouds beyond a projected distance of 50 kpc from M31 is also in conflict with the predictions of recent CDM structure formation simulations. A possible solution to this problem could be ionization of the HVCs as a result of decreasing pressure of the ambient coronal gas at larger distances from M31. A consequence of this scenario would be the presence of hundreds of mainly ionized or pure dark matter satellites around large spiral galaxies like the Milky Way and M31.