Deep imaging of Eridanus II and its lone star cluster

TL;DR

This study provides deep imaging and analysis of Eridanus II, revealing its structural properties, a central star cluster, and stellar populations, confirming it as the least luminous galaxy with a possible central cluster.

Contribution

First deep imaging confirming a central star cluster in Eridanus II and updating its structural and stellar population parameters.

Findings

Eridanus II hosts a central star cluster at its core.

The galaxy has a half-light radius of approximately 280 pc.

No atomic gas detected in deep Green Bank Telescope observations.

Abstract

We present deep imaging of the most distant dwarf discovered by the Dark Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry reaches $\sim$$3$ mag deeper than previous work, and allows us to confirm the presence of a stellar cluster whose position is consistent with Eri II's center. This makes Eri II, at $M_V=-7.1$, the least luminous galaxy known to host a (possibly central) cluster. The cluster is partially resolved, and at $M_V=-3.5$ it accounts for $\sim$$4\%$ of Eri II's luminosity. We derive updated structural parameters for Eri II, which has a half-light radius of $\sim$$280$ pc and is elongated ($\epsilon$$\sim$$0.48$), at a measured distance of $D$$\sim$$370$ kpc. The color-magnitude diagram displays a blue, extended horizontal branch, as well as a less populated red horizontal branch. A central concentration of stars brighter than the old main sequence turnoff hints at a possible intermediate-age ($\sim$$3$ Gyr) population; alternatively, these sources could be blue straggler stars. A deep Green Bank Telescope observation of Eri II reveals no associated atomic gas.