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 , spanning more than a decade in mass .
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 Simulations of The Local Environment ) , a suite of resimulations at various numerical resolution levels ( reaching up to \sim 10 ^ { 4 } { M } _ { \odot } per gas particle ) that use the hydrodynamical code and 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 .
These diagnostics are sensitive to the total mass assumed for the MW-M31 pair ; indeed , the LG satellite population would be quite difficult to reproduce for pair masses as high as indicated by the timing argument .
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 .