A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular Cluster Crater*

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 on the Hubble Space Telescope. Crater's color-magnitude diagram (CMD) extends ∼4 mag below the oldest main-sequence (MS) turnoff. Structurally, we find that Crater has a half-light radius of ∼20 pc and no evidence for tidal distortions. We modelCrater's CMD as a simple stellar population (SSP) and alternatively by solving for its full star formation history. In both cases, Crater is welldescribed by an SSP with an age of ∼7.5 Gyr, a metallicity of [M/H]∼−1.65, a total stellar mass of M 1e4  M , anda luminosity of~-M 5.3V , located at a distance of d∼145 kpc, with modest uncertainties due to differences in the underlying stellar evolution models. We argue that the sparse sampling of stars above the turnoff and subgiant branch are likely to be 1.0-1.4  M blue stragglers and their evolved descendants, as opposed to intermediate-age MS stars. We find thatCrater is an unusually young cluster given its location in the Galaxy's outer halo. We discuss scenarios for Crater's origin, including the possibility of being stripped from the SMC or the accretion from lower-mass dwarfs 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 Galaxy.

The Pristine Dwarf-Galaxy survey – II. In-depth observational study of the faint Milky Way satellite Sagittarius II

“…The satellite is still more extended than the vast majority of MW globular clusters as shown in the bottom left panel of Figure 15, although two of them have a comparable size: Crater (Belokurov et al 2014;Laevens et al 2014) and Terzan 5 (Terzan 1968). These two extended clusters do not, however, share the same metallicity properties as Sgr II: Terzan 5 is a bulge cluster with [Fe/H] > −0.5 and Crater is more metalrich with a systemic metallicity of [Fe/H] ∼ −1.65 (Weisz et al 2016). Our two estimates of the metallicity dispersion of Sgr II both yield similar results and suggest that the satellite was able to retain its gas and form successive generation of stars, thus suggesting the presence of a dark matter halo (Willman & Strader 2012).…”

Reverse engineering the Milky Way

“…Although the high-energy GC Crater does not have an age and metallicity from K19, it has been observed using HST by Weisz et al (2016). They find a good fit to the CMD using a single stellar population model with [α/Fe] = +0.4, age = 7.5 ± 0.4 Gyr, [M/H] = -1.66 ± 0.04 (corresponding to [Fe/H] ∼ -2).…”

Deep Imaging of Eridanus Ii and Its Lone Star Cluster*

“…A significantly younger and more metal-rich population can be ruled out, since it would produce a RGB redder than observed; similarly, for older ages the turnoff would be too faint to match the observed one. We cannot exclude the possibility that these are, at least in part, blue straggler (BS) stars arising from binary systems, which can mimic intermediate-age populations (for a discussion about the "blue plume" in faint MW satellites, see Sand et al 2010;Santana et al 2013;Weisz et al 2016). The ratio of the number of candidate BS stars to the number of blue HB (BHB) stars (computed following Deason et al 2015) is ∼1.7 ± 0.4, consistent with empirical values derived for dwarf galaxies with luminosities similar to Eri II (e.g., Deason et al 2015).…”