WFPC2 observations of the Ursa Minor dwarf spheroidal galaxy

We present our analysis of archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations in F555W (similar to V and F814W (similar to I) of the central region of the Ursa Minor dwarf spheroidal (dSph) galaxy. The V versus V-I color-magnitude diagram features a sparsely populated blue horizontal branch, a steep thin red giant branch, and a narrow subgiant branch. The main sequence reaches approximately 2 mag below the main-sequence turnoff (V-TO(UMi) approximate to 23.27 +/- 0.11 mag) of the median stellar population. We compare the fiducial sequence of Ursa Minor with the fiducial sequence of the Galactic globular cluster M92 (NGC 6341). The excellent match between Ursa Minor and M92 confirms that the median stellar population of the UMi dSph galaxy is metal-poor ([Fe/H](UMi) approximate to [Fe/H](M92) approximate to -2.2 dex) and ancient (age(UMi) approximate to age(M92) approximate to 14 Gyr). The B-V reddening and the absorption in V are estimated to be E(B-V) = 0.03 +/- 0.01 mag and A(V)(UMi) = 0.09 +/- 0.03 mag. A new estimate of the distance modulus of Ursa Minor, (m - M)(o)(UMi) = 19.18 +/- 0.12 mag, has been derived based on fiducial-sequence fitting with M92 [Delta VUMi-M92 = 4.60 +/- 0.03 mag and Delta(V-I)(UMi-M92) = 0.010 +/- 0.005 mag] and the adoption of the apparent V distance modulus for M92 of (m-M)(V)(M92) = 14.67 +/- 0.08 mag. The Ursa Minor dSph galaxy is then at a distance of 69 +/- 4 kpc from the Sun. These HST observations indicate that Ursa Minor has had a very simple star formation history, consisting mainly of a single major burst of star formation about 14 Gyr ago that lasted less than or similar to 2 Gyr. While we may have missed minor younger stellar populations because of the small held of view of the WFPC2 instrument, these observations clearly show that most of the stars in the central region of the Ursa Minor dSph galaxy are ancient. If the ancient Galactic globular clusters such as M92 formed concurrently with the early formation of the Milky Way galaxy itself, then the Ursa Minor dwarf spheroidal is probably as old as the Milky Way.