The Milky Way’s bright satellites as an apparent failure of ΛCDM

We use the Aquarius simulations to show that the most massive subhaloes in galaxy-mass dark matter (DM) haloes in cold dark matter ( CDM) are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have 12 V max 25 km s -1 , CDM simulations predict at least 10 subhaloes with V max > 25 km s -1 . These subhaloes are also among the most massive at earlier times, and significantly exceed the reionization suppression mass back to z 10. No CDM-based model of the satellite population of the Milky Way explains this result. The problem lies in the satellites' densities: it is straightforward to match the observed Milky Way…