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

Understand

We apply a Bayesian method to model multi-epoch radial velocity measurements in the ultra-faint dwarf galaxy Reticulum II, fully accounting for the effects of binary orbital motion and systematic offsets between different spectroscopic datasets.

• We find that the binary fraction of Ret II is higher than 0.5 at the 90% confidence level, if the mean orbital period is assumed to be 30 years or longer.
• Despite this high binary fraction, we infer a best-fit intrinsic dispersion of 2.8$_{-1.2}^{+0.7}$ km/s, which is smaller than previous estimates, but still indicates Ret II is a dark-matter dominated galaxy.
• We likewise infer a $\lesssim$ 1% probability that Ret II's dispersion is due to binaries rather than dark matter, corresponding to the regime $M_{\odot}/L_{\odot} \lesssim$ 2.