Not too big, not too small: the dark haloes of the dwarf spheroidals in the Milky Way

Abstract

We present a new analysis of the Aquarius simulations done in combination with a semi-analytic galaxy formation model. Our goal is to establish whether the subhaloes present in Λ cold dark matter simulations of Milky Way (MW) like systems could host the dwarf spheroidal (dSph) satellites of our Galaxy. Our analysis shows that, contrary to what has been assumed in most previous work, the mass profiles of subhaloes are generally not well fitted by Navarro–Frenk–White models but that Einasto profiles are preferred. We find that for shape parameters α = 0.2–0.5 and vmax = 10–30 km s−1 there is very good correspondence with the observational constraints obtained for the nine brightest dSphs of the MW. However, to explain the internal dynamics of these systems as well as the number of objects of a given circular velocity the total mass of the MW should be ∼8 × 1011 M⊙, a value that is in agreement with many recent determinations, and at the low-mass end of the range explored by the Aquarius simulations. Our simulations show important scatter in the number of bright satellites, even when the Aquarius MW-like hosts are scaled to a common mass, and we find no evidence for a missing population of massive subhaloes in the Galaxy. This conclusion is also supported when we examine the dynamics of the satellites of M31.