An Unexplained Origin for the Unusual Globular Cluster System in the Ultra-diffuse Galaxy FCC 224

TL;DR

This study investigates the globular cluster system of the ultra-diffuse galaxy FCC 224, revealing unique properties and raising questions about its formation, with implications for understanding dark matter and galaxy evolution.

Contribution

The paper provides detailed analysis of FCC 224's globular clusters, highlighting their homogeneous colors, small sizes, and mass segregation, and compares these features with similar galaxies like NGC 1052-DF2 and DF4.

Findings

FCC 224's GCs have homogeneous colors, supporting a single burst formation.

The galaxy exhibits a top-heavy GC luminosity function.

GC sizes are smaller than typical for similar galaxies.

Abstract

We study the quiescent ultra-diffuse galaxy FCC 224 in the Fornax cluster using Hubble Space Telescope (HST) imaging, motivated by peculiar properties of its globular cluster (GC) system revealed in shallower imaging. The surface brightness fluctuation distance of FCC 224 measured from HST is $18.6 \pm 2.7$ Mpc, consistent with the Fornax Cluster distance. We use Prospector to infer the stellar population from a combination of multi-wavelength photometry (HST, ground-based, WISE) and Keck Cosmic Web Imager spectroscopy. The galaxy has a mass-weighted age of $\sim$ 10 Gyr, metallicity [M/H] of $\sim -1.25$ dex, and a very short formation $e$-folding time of $\tau \sim 0.3$ Gyr. Its 12 candidate GCs exhibit highly homogeneous $g_{\rm 475}-I_{\rm 814}$ colors, merely 0.04 mag bluer than the diffuse starlight, which supports a single burst formation scenario for this galaxy. We confirm a top-heavy GC luminosity function, similar to the two dark matter deficient galaxies NGC 1052-DF2 and DF4. However, FCC 224 differs from those galaxies with relatively small GC sizes of $\sim$ 3 pc ($\sim 35\%$ smaller than typical for other dwarfs), and with radial mass segregation in its GC system. We are not yet able to identify a formation scenario to explain all of the GC properties in FCC 224. Follow-up measurements of the dark matter content in FCC 224 will be crucial because of the mix of similarities and differences among FCC 224, DF2, and DF4.