Dynamical Analysis of the Dark Matter and Central Black Hole Mass in the Dwarf Spheroidal Leo I

Abstract

Abstract We measure the central kinematics for the dwarf spheroidal galaxy Leo I using integrated-light measurements and previously published data. We find a steady rise in the velocity dispersion from 300″ into the center. The integrated-light kinematics provide a velocity dispersion of 11.76 ± 0.66 km s−1 inside 75″. After applying appropriate corrections to crowding in the central regions, we achieve consistent velocity dispersion values using velocities from individual stars. Crowding corrections need to be applied when targeting individual stars in high-density stellar environments. From integrated light, we measure the surface brightness profile and find a shallow cusp toward the center. Axisymmetric, orbit-based models measure the stellar mass-to-light ratio, black hole mass, and parameters for a dark matter halo. At large radii it is important to consider possible tidal effects from the Milky Way, so we include a variety of assumptions regarding the tidal radius. For every set of assumptions, models require a central black hole consistent with a mass (3.3 ± 2) × 106 M ⊙. The no-black-hole case for any of our assumptions is excluded at over 95% significance, with 6.4 < Δχ 2 < 14. A black hole of this mass would have significant effects on dwarf galaxy formation and evolution. The dark halo parameters are heavily affected by the assumptions for the tidal radii, with the circular velocity only constrained to be above 30 km s−1. Reasonable assumptions for the tidal radius result in stellar orbits consistent with an isotropic distribution in the velocities. These more realistic models have little need for a dark matter halo.