HUBBLE SPACE TELESCOPE View of the Heart of Ursa Minor

Abstract

Hubble Space Telescope F606W observations to 26th magnitude are used to investigate the stellar distribution in the center of the Ursa Minor dwarf spheroidal galaxy. The central surface density is found to be low, less than 700 stars arcmin^-2, with no sign of a pronounced cusp. The maximum of the surface density is not found at our adopted center but in a ringlike structure some 13" from it. We identify this feature with the subclustering identified by Olszewski & Aaronson, who believed that it was off-center. The currently accepted King's structural parameters, r_c and r_t, of Ursa Minor are found to be unsuitable to represent the stellar surface density near the center. Our star counts are used to obtain a lower limit of the central stellar density without the use of the velocity dispersion. We find a lower limit of 1.7+/-1.1 M_solar pc^-3 from stars that were counted. By extrapolating the mass function to lower masses, we estimate that the true density could increase be a factor of 7.