Paper detail

A comparison of the distribution of satellite galaxies around Andromeda and the results of $Λ$CDM simulations

Ibata et al. (2013) recently reported the existence of a vast thin plane of dwarf galaxies (VTPD) orbiting around Andromeda. We investigate whether such a configuration can be reproduced within the standard cosmological framework and search for similar planes of co-rotating satellite galaxies around Andromeda-like host haloes in data from the Millennium II simulation combined with a semi-analytic galaxy formation model. We apply a baryonic mass cut of $2.8\times 10^4 \text{M}_{\text{sun}}$ for the satellite haloes and restrict the data to a PAndAS like field. If we include the so-called orphan galaxies in our analysis, we find that planes with a rms lower than the VTPD are common in Millennium II. This is partially due to the strongly radially concentrated distribution of orphan galaxies. Excluding part of the orphan galaxies brings the radial distributions of Millennium II satellites into better agreement with the satellite distribution of Andromeda while still producing a significant fraction of planes with a lower rms than the VTPD. We also find haloes in Millennium II with an equal or higher number of co-rotating satellites than the VTPD. This demonstrates that the VTPD is not in conflict with the standard cosmological framework, although a definite answer of this question might require higher resolution cosmological simulations that do not have to consider orphan galaxies. Our results finally show that satellite planes in Millennium II are not stable structures, hence the VTPD might only be a statistical fluctuation of an underlying more spherical galaxy distribution.