Four and one more: The formation history and total mass of globular clusters in the Fornax dSph

TL;DR

This study uses HST data to analyze the star formation history and total mass of Fornax dwarf spheroidal's globular clusters, revealing their ancient origins, metallicity differences, and significant contribution to the galaxy's stellar mass.

Contribution

It provides a detailed star formation history and mass estimates for the globular clusters in Fornax, highlighting their ancient populations and high mass fraction relative to the galaxy.

Findings

Fornax 1, 2, 3 are dominated by populations older than 10 Gyr.

Fornax 4 is younger and more metal-rich, inconsistent with other clusters.

Globular clusters constitute about 2.5% of Fornax's total stellar mass.

Abstract

We have determined the detailed star formation history and total mass of the globular clusters in the Fornax dwarf spheroidal using archival HST WFPC2 data. Colour magnitude diagrams are constructed in the F555W and F814W bands and corrected for the effect of Fornax field star contamination, after which we use the routine Talos to derive the quantitative star formation history as a function of age and metallicity. The star formation history of the Fornax globular clusters shows that Fornax 1, 2, 3 and 5 are all dominated by ancient (>10 Gyr) populations. Cluster Fornax 1,2 and 3 display metallicities as low as [Fe/H]=-2.5 while Fornax 5 is slightly more metal-rich at [Fe/H]=-1.8, consistent with resolved and unresolved metallicity tracers. Conversely, Fornax 4 is dominated by a more metal-rich~([Fe/H]=-1.2) and younger population at 10 Gyr, inconsistent with the other clusters. A lack of stellar populations overlapping with the main body of Fornax argues against the nucleus cluster scenario for Fornax 4. The combined stellar mass in globular clusters as derived from the SFH is (9.57$\pm$0.93)$\times$10$^{5}$ M$_{\odot}$ which corresponds to 2.5$\pm$0.2 percent of the total stellar mass in Fornax. The mass of the four most metal-poor clusters can be further compared to the metal-poor Fornax field to yield a mass fraction of 19.6$\pm$3.1 percent. Therefore, the SFH results provide separate supporting evidence for the unusually high mass fraction of the GCs compared to the Fornax field population.