INTERNAL KINEMATICS OF THE LEO-II DWARF SPHEROIDAL GALAXY

We have obtained radial velocities of 31 red giants in the Leo II dwarf spheroidal (dSph) galaxy using the HIRES echelle spectrograph of the Keck 10 m telescope. These stars were selected using CCD photometry obtained with the Palomar 1.5 m telescope. The radial velocity errors were determined from repeat measurements of Leo II giants and faint radial velocity standards; the average error for the Leo II giants is 2.4 km s-1. The systemic velocity of Leo II is 76.0±1.3 km s-1, and the velocity dispersion is 6.7±1.1 km s-1. Adopting the standard assumptions (mass follows light, isotropy, and equilibrium) we derive a "global" V-band mass-to-light ratio (Mtot/Ltot) = 11.1±3.8, and a central value (ρ0/I0) = 11.6±3.2 (both in solar units). These values are larger than the M/L ratios measured for "local" stellar populations (star clusters or the solar neighborhood). Monte Carlo simulations show that a large population of binaries probably has not inflated a small intrinsic velocity dispersion to the observed value. A model-independent lower limit of the central mass density of Leo II yields ρ0,min=0.074M⊙ pc-3, larger than the central luminous mass density in Leo II for (M/L)1um=2.0. This demands an extensive dark halo in the outer regions of the galaxy to account for the observed dispersion. We conclude that Leo II contains a significant dark matter component. The total mass of the dark halo is 0.9 × 107 M⊙, consistent with the masses inferred in all well-studied dSph galaxies. Because it is remote from the Galaxy, tides cannot plausibly affect the velocity dispersion of Leo II. However, we cannot use these results to rule out the possibility that tides may influence the kinematics of other dSph galaxies found closer to the Milky Way.