Star cluster survival in dark matter halos: An old cluster in Eridanus II?

TL;DR

This study investigates the old star cluster in Eridanus II, revealing it is likely as old and metal-poor as the galaxy's stars, providing insights into dark matter distribution in dwarf galaxies.

Contribution

It presents a Bayesian analysis of Eridanus II's star formation history and suggests the star cluster shares similar age and metallicity with the galaxy's stars, informing dark matter studies.

Findings

Eridanus II's stars are very old (~13.5 Gyr) and metal-poor.

The star cluster likely has a similar age and metallicity as the galaxy's stars.

No recent star formation detected in Eridanus II.

Abstract

The star formation history and the internal dynamics of Milky Way satellite galaxies are often complicated. In the last years, a substantial fraction of the known faint dwarf satellites have been studied. Some of them show embedded stellar substructures, such as star clusters and even globular star clusters. In this work we study Eridanus II, a dwarf spheroidal satellite which hosts a star cluster, using published and archival data from the Hubble Space Telescope Advanced Camera for Surveys. We employ a Bayesian hierarchical method to infer the star formation history of Eridanus II. We find that the bulk of the stars in Eridanus II are very old ($13.5_{-1}^{+0.5}$ Gyr) and quite metal poor ($Z$\,=\,$0.00001$). We do not find any evidence of the presence of an intermediate age or young population in Eri II. We cannot date the embedded star cluster as a separate entity, but we find it likely that the cluster has a similar age and metallicity as the bulk of the stars in Eri II. The existence of an old star cluster in a dark matter dominated old metal poor dwarf galaxy is of major importance to cast light on the dark matter distribution within dwarf galaxies. The existence of intermediate age stars is required by the recent detection of carbon stars in Eri II. Since no recent star formation is detected, {\it blue-straggler} fusions of lower mass stars are the most likely origin of the carbon star progenitors.