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We apply a Bayesian method to model multi-epoch radial velocity measurementsin the ultra-faint dwarf galaxy Reticulum II, fully accounting for the effectsof binary orbital motion and systematic offsets between different spectroscopicdatasets. We find that the binary fraction of Ret II is higher than 0.5 at the90% confidence level, if the mean orbital period is assumed to be 30 years orlonger. Despite this high binary fraction, we infer a best-fit intrinsicdispersion of 2.8$_{-1.2}^{+0.7}$ km/s, which is smaller than previousestimates, but still indicates Ret II is a dark-matter dominated galaxy. Welikewise infer a $\lesssim$ 1% probability that Ret II's dispersion is due tobinaries rather than dark matter, corresponding to the regime$M_{\odot}/L_{\odot} \lesssim$ 2. Our inference of a high close binary fractionin Ret II echoes previous results for the Segue 1 ultra-faint dwarf and isconsistent with studies of Milky Way halo stars that indicate a high closebinary fraction tends to exist in metal-poor environments.

Robust velocity dispersion and binary population modeling of the ultra-faint dwarf galaxy Reticulum II