The Fate of Globular Cluster Substructure: A Kinematic Response to Galaxy Assembly

TL;DR

This study examines how globular cluster kinematics evolve over cosmic time in simulated Milky Way analogues, revealing the limited ability to trace galaxy assembly history solely from current GC motions.

Contribution

It provides new insights into the evolution of GC kinematic properties and their connection to galaxy assembly, highlighting the challenges in reconstructing merger histories from present-day data.

Findings

Most GC progenitor memory is erased by today.

Kinematic properties evolve in both ordered and stochastic ways.

Progenitor halo mass correlates with GC population size and distance.

Abstract

Globular clusters (GCs) are powerful tracers of galaxy assembly, frequently used to identify accreted substructure and reconstruct hierarchical merger histories. With advances in GC formation models and cosmological simulations, we can now better quantify the information about galaxy evolution encoded in present-day GCs. Here, we investigate how GC kinematics evolve over cosmic time and assess the extent to which GCs retain memory of the past of their host galaxy. Using a GC formation model applied to five Milky Way (MW) analogues from the Latte suite of the FIRE-2 simulations, we track the evolution of kinematic properties. At $z=0$, in-situ and ex-situ GCs exhibit substantial overlap in kinematic space, indicating that these populations are not clearly separable. We find that a subset of kinematic properties evolve in an ordered fashion across both in-situ and ex-situ populations, whereas others are dominated by stochastic variations. As a result, by the present day, most memory of the progenitor of an accreted GC is erased and only a few correlations persist. These correlations link progenitor halo mass to the total mass and number of a GC population, and the galactocentric distance of GC substructure to progenitor maximum circular velocity. These results highlight how both deterministic and stochastic processes driven by galaxy evolution shape GC kinematics and demonstrate the limits of reconstructing the assembly history of a galaxy from present-day GC orbits alone.