Estimating the dynamical masses of dwarf galaxies in the presence of binary-star contamination

Abstract

Ultra-faint dwarf galaxies (UFDs) show extreme dynamical mass-to-light ratios of approximately 100-5000 in solar units within the half-light radius, making them critical tests for cosmological models. However, it is a concern whether the line-of-sight (l.o.s.) velocity component of the orbital motion of undetected binary stars is significantly inflating the observed l.o.s. velocity dispersions and, consequently, UFDs dynamical mass estimates. We correct the current estimates of these quantities for UFDs to account for the presence of undetected binaries with single-epoch data. We use the latest binary population models in the solar neighborhood to compute the expected velocity distribution of binary stars. We then convolve this distribution with a Gaussian to model the l.o.s. velocity distribution of UFDs in a mixture model, in which the binary fraction is a free parameter. We apply this methodology to observed UFDs whose dynamical masses are potentially inflated by binaries. In order to generalize to the multi-epoch data case, we compute the velocity distribution of undetected binaries by applying the same cuts to the models as one would apply to the observed data to remove binaries. We find that estimated dynamical masses of UFDs decrease by a factor of 1.5 to 3 once undetected binaries are accounted for. These corrections significantly affect considerations about DM models based on these systems, even challenging the classification of Leo IV, Unions I and Sagittarius II as galaxies. We find that a dedicated multi-epoch campaign spanning one year could substantially mitigate the impact of binaries. Finally, we find that the expected level of binary-star contamination in DM halo density profile inferences from dynamical models of classical dwarf spheroidal galaxies is negligible.