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

TL;DR

This study used radio observations to map neutral hydrogen around Andromeda, finding high-velocity clouds near the galaxy but not beyond 50 kpc, challenging some dark matter models and suggesting ionisation effects.

Contribution

First detailed HI mapping of Andromeda's surroundings revealing the distribution and limits of high-velocity clouds in relation to galaxy formation models.

Findings

HVCs are detected within 50 kpc of M31.

No HVCs are found beyond 50 kpc, contrary to some models.

Ionisation may explain the absence of distant clouds.

Abstract

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-sigma HI mass sensitivity for the warm neutral medium is 8x10^4 solar masses. We can confirm the existence of a population of HVCs with typical HI masses of a few times 10^5 solar masses near the disc of M31. 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 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 ionisation 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 ionised or pure dark-matter satellites around large spiral galaxies like the Milky Way and M31.