The origin of the Milky Way globular clusters

TL;DR

This study uses cosmological simulations to explore the origins of Milky Way globular clusters, confirming their bimodal metallicity distribution and analyzing their environmental evolution and tidal interactions over cosmic time.

Contribution

It provides a self-consistent simulation-based explanation for the bimodal metallicity distribution of globular clusters and their formation environments in the Milky Way.

Findings

Blue, metal-poor clusters form in satellite galaxies accreted onto the Milky Way.

Red, metal-rich clusters form mainly in situ or in merging massive galaxies.

Clusters formed in situ experience stronger tidal forces than accreted ones.

Abstract

We present a cosmological zoom-in simulation of a Milky Way-like galaxy used to explore the formation and evolution of star clusters. We investigate in particular the origin of the bimodality observed in the colour and metallicity of globular clusters, and the environmental evolution through cosmic times in the form of tidal tensors. Our results self-consistently confirm previous findings that the blue, metal-poor clusters form in satellite galaxies which are accreted onto the Milky Way, while the red, metal-rich clusters form mostly in situ or, to a lower extent in massive, self-enriched galaxies merging with the Milky Way. By monitoring the tidal fields these populations experience, we find that clusters formed in situ (generally centrally concentrated) feel significantly stronger tides than the accreted ones, both in the present-day, and when averaged over their entire life. Furthermore, we note that the tidal field experienced by Milky Way clusters is significantly weaker in the past than at present-day, confirming that it is unlikely that a power-law cluster initial mass function like that of young massive clusters, is transformed into the observed peaked distribution in the Milky Way with relaxation-driven evaporation in a tidal field.