The kinematics of globular cluster populations in the E-MOSAICS simulations and their implications for the assembly history of the Milky Way

Abstract

ABSTRACT We present a detailed comparison of the Milky Way (MW) globular cluster (GC) kinematics with the 25 MW-mass cosmological simulations from the E-MOSAICS project. While the MW falls within the kinematic distribution of GCs spanned by the simulations, the relative kinematics of its metal-rich ($[\rm {Fe}/\rm {H}] \gt -1.2$) versus metal-poor ($[\rm {Fe}/\rm {H}] \lt -1.2$) and inner (r < 8 kpc) versus outer (r > 8 kpc) populations are atypical for its mass. To understand the origins of these features, we perform a comprehensive statistical analysis of the simulations, and find 18 correlations describing the assembly of L* galaxies and their dark matter haloes based on their GC population kinematics. The correlations arise because the orbital distributions of accreted and in situ GCs depend on the masses and accretion redshifts of accreted satellites, driven by the combined effects of dynamical fraction, tidal stripping, and dynamical heating. Because the kinematics of in situ/accreted GCs are broadly traced by the metal-rich/metal-poor and inner/outer populations, the observed GC kinematics are a sensitive probe of galaxy assembly. We predict that relative to the population of L* galaxies, the MW assembled its dark matter and stellar mass rapidly through a combination of in situ star formation, more than a dozen low-mass mergers, and 1.4 ± 1.2 early ($z$ = 3.1 ± 1.3) major mergers. The rapid assembly period ended early, limiting the fraction of accreted stars. We conclude by providing detailed quantitative predictions for the assembly history of the MW.