THE INTERNAL KINEMATICS OF THE LEO I DWARF SPHEROIDAL GALAXY - DARK-MATTER AT THE FRINGE OF THE MILKY-WAY

We present radial velocities of 33 red giants in the Leo I dwarf spher oidal galaxy (dSph) obtained from spectra taken with the HIRES echelle spectrograph on the Keck Telescope. These data have a mean precision of 2.2 km s(-1) and lead to estimates of the central velocity dispersi on and systemic velocity of Leo I of 8.8 +/- 1.3 km s(-1) and 287.0 +/ - 1.9 km s(-1), respectively. The systemic velocity confirms past resu lts that Leo I has an unusually large galactocentric velocity, implyin g the presence of a massive dark halo in the Milky Way or an extended dark component pervading the Local Group. The V-band M/L ratio of Leo I is in the range 3.5-5.6. We have produced a set of models that accou nts for the effects of stellar evolution on the global mass-to-light r atio of a composite population. Because Leo I contains a dominant inte rmediate-age population, we find that the V-band mass-to-light ratio o f Leo I would be in the range 6-13 if it were composed exclusively of old stars such as those found in globular clusters. This suggests that Leo I probably does contain a significant dark halo. The mass of this halo is approximately 2 x 10(7) M., similar to the dark halo masses i nferred for all other galactic dSph galaxies. Because Leo I is isolate d and has passed the Milky Way at most once in the past, external tide s could not plausibly have inflated its central dispersion to the obse rved value. We also considered whether modified Newtonian dynamics (MO ND) could account for the internal kinematics of Leo I and conclude th at this alternative gravitational model can account for the Leo I kine matics adequately without requiring a dark halo.