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

TL;DR

This study uses multi-epoch spectroscopy to analyze the stellar kinematics of the ultra-faint dwarf galaxy Triangulum II, developing methods to infer binary star parameters and setting limits on the galaxy's velocity dispersion.

Contribution

It introduces a new methodology for inferring binary orbital parameters from sparse radial velocity data and applies it to Triangulum II.

Findings

Inferred binary orbital parameters with a 296-day period.

Set an upper limit of 3.4 km/s on Triangulum II's velocity dispersion.

Developed a method for analyzing multi-epoch spectroscopic data.

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 $296.0_{-3.3}^{+3.8} \rm~ days$ , semi-major axis $1.12^{+0.41}_{-0.24}\rm~AU$, and a systemic velocity $ -380.0 \pm 1.7 \rm~km ~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 $\sigma_{v} \lesssim 3.4 \rm ~km~s^{-1}$.