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

Abstract

We present a detailed comparison of the Milky Way (MW) globular cluster (GC) kinematics with the 25 Milky Way-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 ($[\rm {Fe}/\rm {H}]>-1.2$) versus metal-poor ($[\rm {Fe}/\rm {H}]<-1.2$), 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.

The Age-Metallicity-Specific Orbital Energy Relation for the Milky Way's Globular Cluster System Confirms the Importance of Accretion for Its Formation

“…Our observation that old, metal-rich globular clusters are more tightly bound to the Milky Way than young, metal-poor globular clusters confirms the importance of accretion for the formation of the Milky Way's globular cluster system. Our inference also supports previous proposals that a large fraction of the Milky Way's metal-poor globular clusters were accreted (e.g., Forbes & Bridges 2010;Forbes 2020;Kruijssen et al 2019aKruijssen et al ,b, 2020Massari et al 2019;Trujillo-Gomez et al 2021).…”

“…(e.g., Kruijssen et al 2019aKruijssen et al ,b, 2020Pfeffer et al 2020;Trujillo-Gomez et al 2021). When applied to the Milky Way, that formalism can quantify the properties of massive satellite galaxies that have since merged with the Milky Way (e.g., Myeong et al 2018;Hughes et al 2019;Kruijssen et al 2019bKruijssen et al , 2020Pfeffer et al 2020;Trujillo-Gomez et al 2021). Moreover, efforts have been made to match the kinematics of clusters to stellar streams associated with known accretion events.…”

The physics governing the upper truncation mass of the globular cluster mass function

“…The simulations have shown that the diversity in age-metallicity relations of Milky Way-mass galaxies results from different assembly histories and can therefore be used to infer such assembly histories (Kruijssen et al 2019a(Kruijssen et al , b , 2020. The simulations have also been shown to reproduce the observed kinematics of the Galactic GC population (Trujillo-Gomez et al 2021 ) and have been used to conclude that GCs associated with stellar streams will be, on average, younger than the GC population not associated with a stellar stream (Hughes et al 2019 ), a result subsequently confirmed through observations of stellar streams in the halo of M31 (Mackey et al 2019 ). Finally, Pfeffer et al ( 2019 ) showed that the simulations reproduce the properties of young cluster populations, and the simulations were subsequently used to predict when and where GCs formed (Reina-Campos et al 2019 ;Keller et al 2020 ).…”

Globular clusters as tracers of the dark matter halo: insights from the E-MOSAICS simulations

“…Additionally, the model has allowed the use of GC populations to trace the formation and assembly history of their host galaxy (e.g. Kruijssen et al 2019a;Hughes et al 2019;Pfeffer et al 2020;Trujillo-Gomez et al 2021). We have recently applied these insights to the GC population of the Milky Way, resulting in the quantitative reconstruction of its merger tree (Kruijssen et al 2019b(Kruijssen et al , 2020.…”