The orbital ellipticity of satellite galaxies and the mass of the Milky Way

We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate the orbits of substructure haloes likely, according to a semi-analytic galaxy formation model, to host luminous satellites. These tend to populate the most massive subhaloes and are on more radial orbits than the majority of subhaloes found within the halo virial radius. One reason for this (mild) kinematic bias is that many low-mass subhaloes have apocentres that exceed the virial radius of the host; they are thus excluded from subhalo samples identified within the virial boundary, reducing the number of subhaloes on radial orbits. Two other factors contributing to the difference in orbital shape between dark and luminous subhaloes are their dynamical evolution after infall, which affects more markedly low-mass (dark) subhaloes, and a weak dependence of ellipticity on the redshift of first infall. The ellipticity distribution of luminous satellites exhibits little halo-to-halo scatter, and it may therefore be compared fruitfully with that of Milky Way satellites. Since the latter depends sensitively on the total mass of the Milky Way we can use the predicted distribution of satellite ellipticities to place constraints on this important parameter. Using the latest estimates of position and velocity of dwarfs compiled from the literature, we find that the most likely Milky Way mass lies in the range 6 × 10¹¹ M_⊙ < M₂₀₀ < 3.1 × 10¹² M_⊙, with a best-fitting value of M₂₀₀ = 1.1 × 10¹² M_⊙. This value is consistent with Milky Way mass estimates based on dynamical tracers or the timing argument.