Leo V: Spectroscopy of a Distant and Disturbed Satellite
Abstract
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 arcmin) and have velocity dispersion 2.4-1.4+2.4 km/s. The other two likely members lie near each other but far from the center (R~13 arcmin ~ 700 pc) and inflate the global velocity dispersion to 3.7-1.4+2.3 km/s. Assuming the five central members are bound, we obtain a dynamical mass of M=3.3-2.5+9.1 x 105Msun (M/LV=75-58+230[M/LV]sun). 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.