Deep imaging of Eridanus II and its lone star cluster
  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.
