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We introduce a comprehensive analysis of multi-epoch stellar line-of-sightvelocities to determine the intrinsic velocity dispersion of the ultrafaintsatellites of the Milky Way. Our method includes a simultaneous Bayesiananalysis of both membership probabilities and the contribution of binaryorbital motion to the observed velocity dispersion within a 14-parameterlikelihood. We apply our method to the Segue 1 dwarf galaxy and conclude thatSegue 1 is a dark-matter-dominated galaxy at high probability with an intrinsicvelocity dispersion of 3.7^{+1.4}_{-1.1} km/sec. The dark matter halo requiredto produce this dispersion must have an average density of 2.5^{+4.1}_{-1.9}solar mass/pc^3 within a sphere that encloses half the galaxy's stellarluminosity. This is the highest measured density of dark matter in the LocalGroup. Our results show that a significant fraction of the stars in Segue 1 maybe binaries with the most probable mean period close to 10 years, but alsoconsistent with the 180 year mean period seen in the solar vicinity at about 1sigma. Despite this binary population, the possibility that Segue 1 is a boundstar cluster with the observed velocity dispersion arising from the orbitalmotion of binary stars is disfavored by the multi-epoch stellar velocity dataat greater than 99% C.L. Finally, our treatment yields a projected(two-dimensional) half-light radius for the stellar profile of Segue 1 of28^{+5}_{-4} pc, in excellent agreement with photometric measurements.

astrophysical journal/the astrophysical journal

A Complete Spectroscopic Survey of the Milky Way satellite Segue 1: Dark  matter content, stellar membership and binary properties from a Bayesian  analysis