In-situ or accreted? Using deep learning to infer the origin of extragalactic globular clusters from observables

TL;DR

This paper presents a deep learning method trained on cosmological simulations to determine whether extragalactic globular clusters formed in their host galaxy or were accreted, achieving high accuracy and robustness to observational uncertainties.

Contribution

The study introduces a supervised deep learning approach that links observable properties of globular clusters to their origin, validated on simulations and real Milky Way data.

Findings

Achieves 89% accuracy in classifying GC origins in simulations.

Successfully identifies accreted GCs in the Milky Way with up to 90% accuracy.

Robust to observational uncertainties and adaptable to different observables.

Abstract

Globular clusters (GCs) are powerful tracers of the galaxy assembly process, and have already been used to obtain a detailed picture of the progenitors of the Milky Way. Using the E-MOSAICS cosmological simulation of a (34.4 Mpc)$^3$ volume that follows the formation and co-evolution of galaxies and their star cluster populations, we develop a method to link the origin of GCs to their observable properties. We capture this complex link using a supervised deep learning algorithm trained on the simulations, and predict the origin of individual GCs (whether they formed in the main progenitor or were accreted from satellites) based solely on extragalactic observables. An artificial neural network classifier trained on $\sim50,000$ GCs hosted by $\sim 700$ simulated galaxies successfully predicts the origin of GCs in the test set with a mean accuracy of $89$ per cent for the objects with [Fe/H]<-0.5 that have unambiguous classifications. The network relies mostly on the alpha-element abundances, metallicities, projected positions, and projected angular momenta of the clusters to predict their origin. A real-world test using the known progenitor associations of the Milky Way GCs achieves up to $90$ per cent accuracy, and successfully identifies as accreted most of the GCs in the inner Galaxy associated to the Kraken progenitor, as well as all the Gaia-Enceladus GCs. We demonstrate that the model is robust to observational uncertainties, and develop a method to predict the classification accuracy across observed galaxies. The classifier can be optimized for available observables (e.g. to improve the accuracy by including GC ages), making it a valuable tool to reconstruct the assembly histories of galaxies in upcoming wide-field surveys.