The aftermath of the Great Collision between our Galaxy and the Large Magellanic Cloud

Abstract

The Milky Way (MW) offers a uniquely detailed view of galactic structure and is often regarded \nas a prototypical spiral galaxy. But recent observations indicate that the MW is atypical: it \nhas an undersized supermassive black hole at its centre; it is surrounded by a very low mass, \nexcessively metal-poor stellar halo; and it has an unusually large nearby satellite galaxy, the \nLarge Magellanic Cloud (LMC). Here, we show that the LMC is on a collision course with the \nMW with which it will merge in 2.4+1.2 −0.8 Gyr (68 per cent confidence level). This catastrophic \nand long-overdue event will restore the MW to normality. Using the EAGLE galaxy formation \nsimulation, we show that, as a result of the merger, the central supermassive black hole will \nincrease in mass by up to a factor of 8. The Galactic stellar halo will undergo an equally \nimpressive transformation, becoming 5 times more massive. The additional stars will come \npredominantly from the disrupted LMC, but a sizeable number will be ejected on to the halo \nfrom the stellar disc. The post-merger stellar halo will have the median metallicity of the LMC, \n[Fe/H] = −0.5 dex, which is typical of other galaxies of similar mass to the MW. At the end \nof this exceptional event, the MW will become a true benchmark for spiral galaxies, at least \ntemporarily.