The Faint Optical Stellar Luminosity Function in the Ursa Minor Dwarf Spheroidal Galaxy

Abstract:
Analyses of their internal stellar kinematics imply that the dwarf spheroidal (dSph) companion galaxies to the Milky Way are among the most dark matter-dominated systems known. Should there be significant dark matter in the form of faint stars in these systems, the stellar luminosity function must be very different from that of a similar metallicity globular cluster, for which there is no evidence for dark matter. We present the faint stellar luminosity function in the Ursa Minor dSph, down to a luminosity corresponding to ~0.45 M_solar, derived from new deep Hubble Space Telescope/WFPC2 data. We find a remarkable similarity between this luminosity function, and inferred initial mass function, and those of the globular cluster M92, a cluster of similar age and metallicity to the Ursa Minor dSph.