How Unusual is the Milky Way’s Assembly History?

Abstract

In the ΛCDM model of structure formation galactic haloes build up by accretion of mass and mergers of smaller haloes. The most recent massive merger event experienced by the Milky Way (MW) halo was the accretion of the Large Magellanic Cloud (LMC; which has a stellar mass of ∼ 109M⊙). Recent analyses of galactic stellar data from the Gaia satellite have uncovered an earlier massive accretion event, the Gaia-Enceladus Sausage (GES), which merged with the MW around 10 Gyr ago. Here, we use the EAGLE cosmological hydrodynamics simulation to study properties of simulated MW-mass haloes constrained to have accretion histories similar to that of the MW, specifically the recent accretion of an “LMC” galaxy and a “GES” merger, with a quiescent period between the GES merger and the infall of the LMC (the “LMC & GES” category). We find that ∼16 per cent of MW-mass haloes have an LMC; ∼5 per cent have a GES event and no further merger with an equally massive object since z = 1; and only 0.65 per cent belong to the LMC & GES category. The progenitors of the MWs in this last category are much less massive than average at early times but eventually catch up with the mean. The LMC & GES category of galaxies naturally end up in the “blue cloud” in the colour-magnitude diagram at z = 0, tend to have a disc morphology and have a larger than average number of satellite galaxies.