The APOSTLE project: Local Group kinematic mass constraints and
  simulation candidate selection
  We use a large sample of isolated dark matter halo pairs drawn from
cosmological N-body simulations to identify candidate systems whose kinematics
match that of the Local Group of Galaxies (LG). We find, in agreement with the
"timing argument" and earlier work, that the separation and approach velocity
of the Milky Way (MW) and Andromeda (M31) galaxies favour a total mass for the
pair of $\sim 5\times 10^{12} \,M_{\odot}$. A mass this large, however, is
difficult to reconcile with the small relative tangential velocity of the pair,
as well as with the small deceleration from the Hubble flow observed for the
most distant LG members. Halo pairs that match these three criteria have
average masses a factor of $\sim 2$ times smaller than suggested by the timing
argument, but with large dispersion. Guided by these results, we have selected
$12$ halo pairs with total mass in the range $1.6$-$3.6 \times
10^{12}\,M_{\odot}$ for the APOSTLE project (A Project Of Simulating The Local
Environment), a suite of hydrodynamical resimulations at various numerical
resolution levels (reaching up to $\sim10^{4}\,M_{\odot}$ per gas particle)
that use the subgrid physics developed for the EAGLE project. These simulations
reproduce, by construction, the main kinematics of the MW-M31 pair, and produce
satellite populations whose overall number, luminosities, and kinematics are in
good agreement with observations of the MW and M31 companions. The APOSTLE
candidate systems thus provide an excellent testbed to confront directly many
of the predictions of the $\Lambda$CDM cosmology with observations of our local
Universe.
