Magellan/M2FS Spectroscopy of the Reticulum 2 Dwarf Spheroidal Galaxy

Matthew G. Walker; Mario Mateo; Edward W. Olszewski; John I. Bailey; Sergey E. Koposov; Vasily Belokurov; N. Wyn Evans

doi:10.1088/0004-637X/808/2/108

paper

Magellan/M2FS Spectroscopy of the Reticulum 2 Dwarf Spheroidal Galaxy

arXiv:1504.03060 · doi:10.1088/0004-637X/808/2/108

Abstract

We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 182 stellar targets along the line of sight to the newly-discovered `ultrafaint' object Reticulum 2 (Ret 2). For 38 of these targets, the spectra are sufficient to provide simultaneous estimates of line-of-sight velocity ( $v_{los}$ , median random error $δ_{v_{los}} = 1.3$ km s $^{- 1}$ ), effective temperature ( $T_{eff}$ , $δ_{T_{eff}} = 464$ K), surface gravity ( $lo g g$ , $δ_{logg} = 0.54$ dex) and iron abundance ([Fe/H], $δ_{[Fe/H]} = 0.45$ dex). We use these results to confirm 18 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of $σ_{v_{los}} = 3. 6_{- 0.6}^{+ 0.9}$ km s $^{- 1}$ about a mean of $⟨ v_{los} ⟩ = 64. 8_{- 1.0}^{+ 1.1}$ km s $^{- 1}$ in the solar rest frame ( $\sim - 90.9$ km s $^{- 1}$ in the Galactic rest frame), and a metallicity dispersion of $σ_{[Fe/H]} = 0.5 0_{- 0.13}^{+ 0.17}$ dex about a mean of $⟨ [Fe/H] ⟩ = - 2.6 7_{- 0.34}^{+ 0.34}$ . These estimates marginalize over possible velocity and metallicity gradients, which are consistent with zero. Our results place Ret 2 on chemodynamical scaling relations followed by the Milky Way's dwarf-galactic satellites. Under assumptions of dynamic equilibrium and negligible contamination from binary stars---both of which must be checked with deeper imaging and repeat spectroscopic observations---the estimated velocity dispersion suggests a dynamical mass of $M (R_{h}) \approx 5 R_{h} σ_{v_{los}}^{2} / (2 G) = 2. 4_{- 0.8}^{+ 1.3} \times 1 0^{5}$ $M_{⊙}$ enclosed within projected halflight radius $R_{h} \sim 32$ pc, with mass-to-light ratio $\approx 2 M (R_{h}) / L_{V} = 46 2_{- 157}^{+ 264}$ in solar units.

submitted to The Astrophysical Journal

Magellan/M2FS Spectroscopy of the Reticulum 2 Dwarf Spheroidal Galaxy

Abstract

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