The spatial distribution of the Milky Way and Andromeda satellite galaxies

There are two fundamentally different physical origins of faint satellite galaxies: cosmological substructures that contain shining baryons and the fragmentation of gas-rich tidal arms thrown out from interacting galaxies during hierarchical structure formation. The latter tidal dwarf galaxies (TDGs) may form populations with correlated orbital angular momenta about their host galaxies. The existence of TDGs is a stringent necessity because they arise as a result of fundamental physical principles. We determine the significance of the apparent disc-like distribution of Milky Way (MW) satellite galaxies. The distribution of the MW satellites is found to be inconsistent with an isotropic or prolate dark matter substructure distribution at a 99.5 per cent level including the recently discovered Ursa Major and Canes Venatici dwarf spheroidal galaxies. The distribution is extremely oblate and inclined by about 88° with respect to the the MW disc. We also apply the methods to Andromeda's (M31) satellite galaxies using two recently published data sets. It cannot be excluded that the whole population of M31 companions is drawn randomly from an isotropic parent distribution. However, two subsamples of Andromeda satellites are identified which have disc-like features. A kinematically motivated subsample of eight Andromeda satellites form a pronounced disc-like distribution in both data sets. The existence of this disc would be inconsistent with a cold dark matter parent distribution of subhaloes if the disc is rotationally supported. The M31 satellite distribution is inclined by about 59° with respect to the M31 disc, and has virtually the same orientation as the disc derived for the whole M31 satellite sample. We present a new geometric method to set restrictions on possible locations of angular momentum vectors for Andromeda satellites. Our conclusion is that both, the MW and M31, may indeed have satellite galaxies derived from TDGs. Further, both host-discs and both identified discs-of-satellites are highly inclined relative to the supergalactic plane. The discs-of-satellites therefore cannot be created from individual accretion events from the supergalactic plane further supporting the possibility that they are of TDG origin.