The Local Group of galaxies offer some of the most discriminating tests of models of cosmic structure formation. For example, observations of the Milky Way (MW) and Andromeda satellite populations appear to be in disagreement with N-body simulations of the "Lambda Cold Dark Matter" (ΛCDM) model: there are far fewer satellite galaxies than substructures in cold dark matter halos (the "missing satellites" problem); dwarf galaxies seem to avoid the most massive substructures (the "too-big-tofail" problem); and the brightest satellites appear to orbit their host galaxies on a thin plane (the "planes of satellites" problem). Here we present results from Apostle (A Project Of Simulating The Local Environment), a suite of cosmological hydrodynamic simulations of twelve volumes selected to match the kinematics of the Local Group (LG) members. Applying the Eagle code to the LG environment, we find that our simulations match the observed abundance of LG galaxies, including the satellite galaxies of the MW and Andromeda. Due to changes to the structure of halos and the evolution in the LG environment, the simulations reproduce the observed relation between stellar mass and velocity dispersion of individual dwarf spheroidal galaxies without necessitating the formation of cores in their dark matter profiles. Satellite systems form with a range of spatial anisotropies, including one similar to that of the MW, confirming that such a configuration is not unexpected in ΛCDM. Finally, based on the observed velocity dispersion, size, and stellar mass, we provide new estimates of the maximum circular velocity for the halos of nine MW dwarf spheroidals.

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We determine the Milky Way (MW) mass profile inferred from fitting physically motivated models to the Gaia DR2 Galactic rotation curve and other data. Using various hydrodynamical simulations of MW-mass haloes, we show that the presence of baryons induces a contraction of the dark matter (DM) distribution in the inner regions, r 20 kpc. We provide an analytic expression that relates the baryonic distribution to the change in the DM halo profile. For our galaxy, the contraction increases the enclosed DM halo mass by factors of roughly 1.3, 2 and 4 at radial distances of 20, 8 and 1 kpc, respectively compared to an uncontracted halo. Ignoring this contraction results in systematic biases in the inferred halo mass and concentration. We provide a best-fitting contracted NFW halo model to the MW rotation curve that matches the data very well. The best-fit has a DM halo mass, M DM 200 = 0.99 +0.18 −0.20 ×10 12 M , and concentration before baryon contraction of 8.2 +1.7 −1.5 , which lie close to the median halo mass-concentration relation predicted in ΛCDM. The inferred total mass, M total 200 = 1.12 +0.20 −0.22 × 10 12 M , is in good agreement with recent measurements. The model gives a MW stellar mass of 4.99 +0.34 −0.50 × 10 10 M , of which 60% is contained in the thin stellar disc, with a bulge-to-total ratio of 0.2. We infer that the DM density at the Solar position is ρ DM = 9.0 +0.5 −0.4 × 10 −3 M pc −3 ≡ 0.34 +0.02 −0.02 GeV cm −3 . The rotation curve data can also be fitted with an uncontracted NFW halo model, but with very different DM and stellar parameters. The observations prefer the physically motivated contracted NFW halo, but the measurement uncertainties are too large to rule out the uncontracted NFW halo.

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 ∼ 5 × 10 12 M ⊙ . 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 ∼ 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 × 10 12 M ⊙ for the APOSTLE project (A Project Of Simulations of The Local Environment), a suite of resimulations at various numerical resolution levels (reaching up to ∼ 10 4 M ⊙ per gas particle) that use the hydrodynamical code and subgrid physics developed for the EA-GLE 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 ΛCDM cosmology with observations of our local Universe.