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

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

2008-08-27

arxiv.org/abs/0808.3611v1

Astronomy and Astrophysics

Astrophysics

21 pages, 15 figures, accepted for publication in MNRAS

Scientific paper

10.1111/j.1365-2966.2008.13858.x

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.

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