SIMBAD references

We compare spherically averaged radial number counts of bright ( ≳ 105L_☉) dwarf satellite galaxies within 400kpc of the Milky Way (MW) and M31 and find that the MW satellites are much more centrally concentrated. Remarkably, the two satellite systems are almost identical within the central 100kpc, while M31 satellites outnumber MW satellites by about a factor of 4 at deprojected distances spanning 100-400kpc. We compare the observed distributions to those predicted for Λ cold dark matter (ΛCDM) subhaloes using a suite of 44 high-resolution ∼ 1012M_☉ halo zoom simulations, 22 of which are in pairs like the MW and M31. We find that the radial distribution of satellites around M31 is fairly typical of those predicted for subhaloes, while the MW's distribution is more centrally concentrated than any of our simulated ΛCDM haloes. One possible explanation is that our census of bright ( ≳ 105L_☉) MW dwarf galaxies is significantly incomplete beyond ∼ 100kpc of the Sun. If there were ∼ 8-20 more bright dwarfs orbiting undetected at 100-400kpc distance, then the MW's radial distribution would fall within the range expected from subhalo distributions and also look very much like the known M31 system. We use our simulations to demonstrate that there is enough area left unexplored by the Sloan Digital Sky Survey and its extensions that the discovery of ∼ 10 new bright dwarfs is not implausible given the expected range of angular anisotropy of subhaloes in the sky.