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 dSph galaxy obtained from spectra taken with the HIRES echelle spectrograph on the Keck I telescope. These data have a mean precision of 2.2 km s and lead to estimates of the central velocity dispersion and systemic velocity of Leo I of 8.8 ± 1.3 km s, and 287.0± 1.9 km s, respectively. The systemic velocity confirms past results that Leo I has an unusually large galactocentric velocity, implying 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 account for the effects of stellar evolution on the global mass-to-light ratio of a composite population. Because Leo I contains a dominant intermediate-age population, we find that the V-band mass-to-light ratio of Leo I would be in the range 6-13 if it were composed exclusively of old stars such as found in globular clusters. This suggests that Leo I probably does contain a significant dark halo. The mass of this halo is approximately 2× 10 M , similar to the dark halo masses inferred for all other Galactic dSph galaxies. Because Leo I is isolated and has passed the Milky Way at most once in the past, external tides could not plausibly have inflated its central dispersion to the observed value. We also considered whether MOdified Newtonian Dynamics (MOND) could account for the internal kinematics of Leo I and conclude that this alternative gravitational model can account for the Leo I kinematics adequately without requiring a dark halo. The agreement with MOND is particularly good if the velocity dispersion exhibits some anisotropy or the underlying stellar mass function is only slightly different than a classical Salpeter law.