Leo V: Spectroscopy of a Distant and Disturbed Satellite

We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3) and have velocity dispersion 2.4 −1.4 km s. The other two likely members lie near each other but far from the center (R ∼ 13 ∼ 700 pc) and inflate the global velocity dispersion to 3.7 −1.4 km s . Assuming the five central members are bound, we obtain a dynamical mass of M = 3.3 −2.5 × 10 M⊙ (M/LV = 75 +230 −58 [M/LV ]⊙). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]= −2.0 ± 0.2 dex. Thus with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal-rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D ∼ 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial. Subject headings: galaxies: dwarf — galaxies: kinematics and dynamics — (galaxies:) Local Group — (cosmology:) dark matter —