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

Research output: Contribution to journalArticlepeer-review

63 Scopus citations


We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 182 stellar targets along the line of sight (LOS) to the newly discovered "ultrafaint" object Reticulum 2 (Ret 2). For 37 of these targets, the spectra are sufficient to provide simultaneous estimates of LOS velocity (νlos, median random error δνlos= 1.4 km s-1), effective temperature (Teff, Δ Teff K), surface gravity (loh g, δlog g = 0.63 dex), and iron abundance ([Fe/H], δ[Fe/H] = 0.47 dex). We use these results to confirm 17 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of σνlos = 3.60.7+1.0km s-1 about a mean of 〈νlos〉 = 64.3-1.2+1.2 km s-1 in the solar rest frame ( ∼ -90.9 km s-1 in the Galactic rest frame), and a metallicity dispersion of σ [Fe/H] = 0.49-0.140.19 dex about a mean of 〈 [Fe/H]〉= -2.58-0.33+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(Rh)≈5Rhσνlos;2/(2G) = 2.4-0.8+1.4 × 105 Menclosed within projected halflight radius Rh ∼32 pc, with mass-to-light ratio ≈2M (Rh)/LV = 467-168+286in solar units.

Original languageEnglish (US)
Article number108
JournalAstrophysical Journal
Issue number2
StatePublished - Aug 1 2015


  • Local Group
  • galaxies: dwarf
  • galaxies: individual (Reticulum 2)
  • galaxies: kinematics and dynamics
  • methods: data analysis
  • techniques: spectroscopic

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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