Abstract
We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The HST-based colormagnitude diagram (CMD) of Crater extends 4 magnitudes below the oldest main sequence turnoff, providing excellent leverage on Crater’s physical properties. Structurally, we find that Crater has a half-light radius of 20 pc and shows no evidence for tidal distortions. We model the CMD of Crater under the assumption of it being a simple stellar population and alternatively by solving for its full star formation history. In both cases, Crater is well-described by a simple stellar population with an age of 7.5 Gyr, a metallicity of [M/H] -1.65, a total stellar mass of M? 1e4 M , a luminosity of MV -5:3, located at a distance of d 145 kpc, with modest uncertainties in these properties due to differences in the underlying stellar evolution models. We argue that the sparse sampling of stars above the turnoff and sub-giant branch are likely to be 1.0-1.4 M binary star systems (blue stragglers) and their evolved descendants, as opposed to intermediate age main sequence stars. Confusion of these populations highlights a substantial challenge in accurately characterizing sparsely populated stellar systems. Our analysis shows that Crater is not a dwarf galaxy, but instead is an unusually young cluster given its location in the Milky Way’s very outer stellar halo. Crater is similar to SMC cluster Lindsay 38, and its position and velocity are in good agreement with observations and models of the Magellanic stream debris, suggesting it may have accreted from the Magellanic Clouds. However, its age and metallicity are also in agreement with the age-metallicity relationships of lower mass dwarf galaxies such as Leo I or Carina. Despite uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently than z 1 (8 Gyr ago), providing an important new constraint on the accretion history of the Milky Way.