A V-CCD AND I-CCD MOSAIC SURVEY OF THE URSA-MINOR DWARF SPHEROIDAL GALAXY

We discuss a Johnson-Cousins V- and I-band CCD mosaic survey of the Ur sa Minor dwarf spheroidal galaxy (dSph) to V similar to 22. We covered UMi with 27 10'.5 x 10'.5 overlapping CCD frames, each frame consisti ng of two 300 s exposures in each of V and I. We also observed several regions similar to 3 degrees from UMi to obtain an estimate of contam ination by galaxies and Galactic stars. We report the first H-R diagra m of an entire dSph. Separation of dwarf stars from foreground stars b y color allows a robust estimation of the structural parameters of UMi . We examine earlier evidence of two lumps in the UMi stellar distribu tion. We detect a statistically significant asymmetry in the stellar d istribution of UMi along the major axis. Structure in the stellar dist ribution of UMi might indicate a tidal origin for UMi's high observed mass-to-light ratio. We demonstrate a technique for obtaining the abso lute magnitude of UMi from horizontal-branch star counts compared with M92; this method of luminosity estimation is independent of the dista nce to UMi. We obtain M-V = -8.87 +/- sigma(Mv), where sigma(Mv) = 0.1 4 if M92 is a perfect calibrator and sigma(Mv) less than or similar to 0.35 is an upper bound on the error arising from differences in stell ar populations. Our value for M-v is consistent with earlier measureme nts and has a smaller uncertainty.