2019MNRAS.487.2961M

We apply a Bayesian method to model multi-epoch radial velocity measurements in the ultrafaint dwarf galaxy Reticulum II, fully accounting for the effects of binary orbital motion and systematic offsets between different spectroscopic data sets. We find that the binary fraction of Ret II is higher than 0.5 at the 90 per cent confidence level, if the mean orbital period is assumed to be 30 yr or longer. Despite this high-binary fraction, we infer a best-fitting intrinsic dispersion of 2.8_–1.2^+0.7 km s^–1, which is smaller than the previous estimates but still indicates Ret II is a dark matter dominated galaxy. We likewise infer a <= 1 per cent probability that Ret II's dispersion is due to binaries rather than dark matter (which would correspond to the regime M/L ≲2 M_☉/L☉). Our inference of a high-close binary fraction in Ret II echoes previous results for the Segue 1 ultrafaint dwarf and is consistent with studies of Milky Way halo stars that indicate a high-close binary fraction tends to exist in metal-poor environments.