Stellar kinematics of dwarf galaxies from multi-epoch spectroscopy: application to Triangulum II

Abstract

We present new MMT/Hectochelle spectroscopic measurements for 257 stars observed along the line of sight to the ultra-faint dwarf galaxy Triangulum II. Combining with results from previous Keck/DEIMOS spectroscopy, we obtain a sample that includes 16 likely members of Triangulum II, with up to 10 independent redshift measurements per star. To this multi-epoch kinematic data set we apply methodology that we develop in order to infer binary orbital parameters from sparsely sampled radial velocity curves with as few as two epochs. For a previously-identified (spatially unresolved) binary system in Tri~II, we infer an orbital solution with period $296.0_{-3.3}^{+3.8}\mathrm{days}$ , semi-major axis $1.12_{-0.24}^{+0.41}\mathrm{AU}$, and a systemic velocity $-380.0\pm 1.7\mathrm{km}{\mathrm{s}}^{-1}$ that we then use in the analysis of Tri~II's internal kinematics. Despite this improvement in the modeling of binary star systems, the current data remain insufficient to resolve the velocity dispersion of Triangulum II. We instead find a 95% confidence upper limit of ${\sigma }_v\lesssim 3.4\mathrm{km}{\mathrm{s}}^{-1}$.

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