Eridanus II: A Fossil from Reionization with an Off-Center Star Cluster
  We present deep Hubble Space Telescope (HST) photometry of the ultra-faint
dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of
M_V = -7.1, is located at a distance of 339 kpc, just beyond the virial radius
of the Milky Way. We determine the star formation history of Eri II and measure
the structure of the galaxy and its star cluster. We find that a star formation
history consisting of two bursts, constrained to match the spectroscopic
metallicity distribution of the galaxy, accurately describes the Eri II stellar
population. The best-fit model implies a rapid truncation of star formation at
early times, with >80% of the stellar mass in place before z~6. A small
fraction of the stars could be as young as 8 Gyr, but this population is not
statistically significant; Monte Carlo simulations recover a component younger
than 9 Gyr only 15% of the time, where they represent an average of 7 +/- 4% of
the population. These results are consistent with theoretical expectations for
quenching by reionization. The HST depth and angular resolution enable us to
show that Eri II's cluster is offset from the center of the galaxy by a
projected distance of 23 +/- 3 pc. This offset could be an indication of a
small (~50-75 pc) dark matter core in Eri II. Moreover, we demonstrate that the
cluster has a high ellipticity of 0.31 +0.05/-0.06 and is aligned with the
orientation of Eri II within 3 +/- 6 degrees, likely due to tides. The stellar
population of the cluster is indistinguishable from that of Eri II itself.
