Dark Matter in the Local Group: The Dark Haloes of the Milky Way, M31 and the Draco dSph

Abstract.

Mass estimates of the dark matter haloes of the Milky Way and M31 based on the latest observational data sets of tracer objects outside 20kpc are presented. The most likely masses are found to be 2.0_-1.8 ^+3.8 x 10¹²M$_{\odot}$ and 1.0_-0.6 ^+1.8 x 10¹²M$_{\odot}$, respectively. We conclude that there is no dynamical evidence that M31 is more massive than the Milky Way. The results of a detailed study of the internal dynamics of the Draco dwarf spheroidal (dSph) galaxy based on a new data set of stellar velocities are presented, providing the first clear evidence of an extended dark matter halo in Draco. We obtain a best-fit mass within 3 core radii of 8⁺³ _-2 x 10⁷M$_{\odot}$ implying a V-band M/L ratio of $440 \pm 240$M$_{\odot}/$L$_{\odot}$. Both tidal disruption and modified gravity (MOND) models are shown to be unsatisfactory explanations of the data.