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 main host; they are thus excluded from subhalo
samples identified within the virial boundary, reducing the number of subhalos
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 $6 \times 10^{11} M_{\odot} < M_{200} < 3.1 \times
10^{12} M_{\odot}$, with a best fit value of $M_{200} = 1.1 \times 10^{12}
M_{\odot}$. This value is consistent with Milky Way mass estimates based on
dynamical tracers or the timing argument.
