Are Some Milky Way Globular Clusters Hosted by Undiscovered Galaxies?

TL;DR

This study suggests that some of the Milky Way's outer halo globular clusters may reside in undetected low-mass subhalos, extending the known relationship between galaxy mass and globular cluster count to smaller scales.

Contribution

It extends the galaxy mass-GC relationship to lower masses and proposes that some MW GCs are hosted by undetected subhalos with minimal stellar content.

Findings

Milky Way subhalos could host 5 to 31 GCs.

Approximately 90% of these GCs are in subhalos less massive than Eri II.

Some MW GCs likely reside in undetected low-mass subhalos.

Abstract

The confirmation of a globular cluster (GC) in the recently discovered ultrafaint galaxy Eridanus II (Eri II) motivated us to examine the question posed in the title. After estimating the halo mass of Eri II using a published stellar mass - halo mass relation, the one GC in this galaxy supports extending the relationship between the number of GCs hosted by a galaxy and the galaxy's total mass about two orders of magnitude in stellar mass below the previous limit. For this empirically determined specific frequency of between 0.06 and 0.39 globular clusters per 10$^9$ $M_\odot$ of total mass, the surviving Milky Way (MW) subhalos with masses smaller than $10^{10} M_\odot$ could host as many as 5 to 31 GCs, broadly consistent with the actual population of outer halo MW GCs, although matching the radial distribution in detail remains a challenge. Using a subhalo mass function from published high resolution numerical simulations and a Poissonian model for populating those halos with the aforementioned empirically constrained frequency, we find that about 90$\%$ of these GCs lie in lower-mass subhalos than that of Eri II. From what we know about the stellar mass-halo mass function, the subhalo mass function, and the mass-normalized GC specific frequency, we conclude that some of the MW's outer halo GCs are likely to be hosted by undetected subhalos with extremely modest stellar populations.