Great Balls of FIRE II: The evolution and destruction of star clusters across cosmic time in a Milky Way-mass galaxy

TL;DR

This study uses detailed star-by-star simulations to explore how star clusters form, evolve, and are destroyed over cosmic time within a galaxy similar to the Milky Way, revealing the influence of galactic history on globular cluster properties.

Contribution

First star-by-star N-body models of massive star clusters within a cosmological galaxy simulation, linking cluster evolution to galaxy formation history.

Findings

Predicted 148 globular clusters in a Milky Way-like galaxy.

Younger clusters are less massive and more core-collapsed due to galactic environment effects.

Cluster properties are shaped by both internal dynamics and host galaxy evolution.

Abstract

The current generation of galaxy simulations can resolve individual giant molecular clouds, the progenitors of dense star clusters. But the evolutionary fate of these young massive clusters, and whether they can become the old globular clusters (GCs) observed in many galaxies, is determined by a complex interplay of internal dynamical processes and external galactic effects. We present the first star-by-star $N$-body models of massive ($N\sim10^5-10^7$) star clusters formed in a FIRE-2 MHD simulation of a Milky Way-mass galaxy, with the relevant initial conditions and tidal forces extracted from the cosmological simulation. We select 895 ($\sim 30\%$) of the YMCs with $ > 6\times10^4M_{\odot}$ from Grudi\'c et al.~2022 and integrate them to $z=0$ using the Cluster Monte Carlo Code, \texttt{CMC}. This procedure predicts a MW-like system with 148 GCs, predominantly formed during the early, bursty mode of star formation. Our GCs are younger, less massive, and more core-collapsed than clusters in the Milky Way or M31. This results from the assembly history and age-metallicity relationship of the host galaxy: younger clusters are preferentially born in stronger tidal fields and initially retain fewer stellar-mass black holes, causing them to lose mass faster and reach core collapse sooner than older GCs. Our results suggest that the masses and core/half-light radii of GCs are shaped not only by internal dynamical processes, but also by the specific evolutionary history of their host galaxies. These results emphasize that $N$-body studies with realistic stellar physics are crucial to understanding the evolution and present-day properties of GC systems.