A high occurrence of nuclear star clusters in faint Coma galaxies, and the roles of mass and environment

TL;DR

This study reveals a high occurrence of nuclear star clusters in faint dwarf galaxies within the Coma cluster, highlighting the influence of galaxy mass and environment on NSC formation.

Contribution

It introduces a hierarchical Bayesian model to analyze the nucleation fraction's dependence on luminosity and environment in galaxy clusters.

Findings

Half of the dwarfs at M_I ≈ -13 mag host NSCs.

Nucleation probability's dependence on luminosity is nearly universal.

Environmental factors significantly affect the fraction of nucleated galaxies.

Abstract

We use deep high resolution \textit{HST/ACS} imaging of two fields in the core of the Coma cluster to investigate the occurrence of nuclear star clusters (NSCs) in quiescent dwarf galaxies as faint as $M_{I} = -10$ mag. We employ a hierarchical Bayesian logistic regression framework to model the faint end of the nucleation fraction ($f_{n}$) as a function of both galaxy luminosity and environment. We find that $f_n$ is remarkably high in Coma: at $M_{I} \approx -13$ mag half of the cluster dwarfs still host prominent NSCs. Comparison with dwarf systems in nearby clusters and groups shows that, within the uncertainties, the rate at which the probability of nucleation varies with galaxy luminosity is nearly universal. On the other hand, the fraction of nucleated galaxies at fixed luminosity does exhibit an environmental dependence. More massive environments feature higher nucleation fractions and fainter values of the half-nucleation luminosity, which roughly scales with host halo virial mass as $L_{I,f_{n50}} \propto \mathcal{M}_{200}^{-0.2}$. Our results reinforce the role of galaxy luminosity/mass as a major driver of the efficiency of NSC formation and also indicate a clear secondary dependence on the environment, hence paving the way to more refined theoretical models.