Did the Gaia Enceladus/Sausage merger form the Milky Way’s bar?

Abstract

ABSTRACT The Milky Way’s last significant merger, the Gaia Enceladus/Sausage (GES), is thought to have taken place between $8\hbox{ and }11\, \mathrm{Gyr}$ ago. Recent studies in the literature suggest that the bar of the Milky Way is rather old, indicating that it formed at a similar epoch to the GES merger. We investigate the possible link between these events using one of the Auriga cosmological simulations that has salient features in common with the Milky Way, including a last significant merger with kinematic signatures resembling that of the GES. In this simulation, the GES-like merger event triggers tidal forces on the disc, gas inflows, and a burst of star formation, with the formation of a bar occurring within $1\, \mathrm{Gyr}$ of the first pericentre. To highlight the effects of the merger, we rerun the simulation from z = 4 with the progenitors of the GES-like galaxy removed well before the merger time. The consequence is a delay in bar formation by around $2\, \mathrm{Gyr}$, and this new bar forms without any significant external perturbers. We conclude that this Milky Way-like simulation shows a route to the real Milky Way’s bar forming around the epoch of the GES merger due to tidal forces on its first pericentre. We explore all Auriga galaxies with GES-like merger events, and find that those with stellar mass ratios below $10{{\, \rm per\, cent}}$ form bars within $1\, \mathrm{Gyr}$ of the merger, while bar formation is delayed in the more massive merger scenarios. These include the four oldest bars in the simulation suite. Lastly, we note some later morphological differences between the disc of the original simulation and our rerun, in particular that the latter does not grow radially for the final $7\, \mathrm{Gyr}$. Our study suggests that the GES may therefore be responsible for the formation of the Milky Way’s bar, as well as for the build-up of its extended disc.