Globular cluster ages and their relation to high-redshift stellar cluster formation times from different globular cluster models

TL;DR

This study compares six globular cluster formation models to observations, revealing differences in predicted ages and formation redshifts, and highlighting uncertainties about the origins of young high-redshift clusters.

Contribution

It provides the first side-by-side comparison of multiple GC formation models against observed GC ages and formation redshifts in Milky Way-like galaxies.

Findings

All models produce GCs older than 10 Gyr, consistent with observations.

Four models predict younger GCs than observed in the Milky Way.

Models differ on the presence of old GCs in MW-mass galaxies.

Abstract

The formation details of globular clusters (GCs) are still poorly understood due to their old ages and the lack of detailed observations of their formation. A large variety of models for the formation and evolution of GCs have been created to improve our understanding of their origins, based on GC properties observed at z=0. We present the first side-by-side comparison of six current GC formation models with respect to their predictions for the GC ages and formation redshifts in Milky Way (MW)-like galaxies. We find that all the models are capable of forming most of the surviving GCs at more than 10 Gyr ago, in general agreement with the observation that most GCs are old. However, the measured MW GC ages are still systematically older than those predicted in the galaxies of four of the models. Investigating the variation of modelled GC age distributions for general MW-mass galaxies, we find that some of the models predict that a significant fraction of MW-mass galaxies would entirely lack a GC population older than 10 Gyr, whereas others predict that all MW-mass galaxies have a significant fraction of old GCs. This will have to be further tested in upcoming surveys, as systems without old GCs in that mass range are currently not known. Finally, we show that the models predict different formation redshifts for the oldest surviving GCs, highlighting that models currently disagree about whether the recently observed young star clusters at high redshifts could be the progenitors of today's GCs.