Stellar Kinematics in the Remote Leo II Dwarf Spheroidal Galaxy--Another Brick in the Wall

Abstract

We present the projected velocity dispersion profile for the remote (d = 233 kpc) Galactic dwarf spheroidal (dSph) galaxy Leo II, based on 171 discrete stellar radial velocities that were obtained from medium-resolution spectroscopy using the FLAMES/GIRAFFE spectrograph at the European Southern Observatory, Chile. The dispersion profile of those stars with good membership probabilities is essentially flat, with an amplitude of 6.6 ± 0.7 km s-1 over the full radial extent of our data, which probe to the stellar boundary of this galaxy. We find no evidence of any significant apparent rotation or velocity asymmetry, which suggests that tidal effects cannot be invoked to explain Leo II's properties. From basic mass modeling, employing the Jeans equation, we derive a mass out to the limiting radius of (2.7 ± 0.5) × 107 M⊙ and a global mass-to-light ratio of 27-45 in solar units, depending on the adopted total luminosity. A cored halo profile and a mild amount of tangential velocity anisotropy is found to account well for Leo II's observed kinematics, although we cannot exclude the possibility of a cusped halo with radially varying velocity anisotropy. All in all, this galaxy exhibits dark matter properties that appear to be concordant with the other dSph satellites of the Milky Way, namely, a halo mass profile that is consistent with a central core and a total mass that is similar to the common mass scale seen in other dSphs.