A Suprime-Cam study of the stellar population of the Ursa Major I dwarf spheroidal galaxy

We present deep and wide V, I CCD photometry of the Ursa Major I (UMa I) dwarf spheroidal galaxy (dSph) in the Local Group. The images of the galaxy were taken with the Subaru/Suprime-Cam wide field camera, covering a field of 34' x 27' located at the centre of the galaxy. The colour-magnitude diagram (CMD) of the UMa I dSph shows a steep and narrow red giant branch (RGB), blue and red horizontal branch (HB), and main sequence (MS) stars. A well-defined main sequence turn-off (MSTO) is found to be located at V-0,V-MSTO similar to 23.5 mag. The distance modulus is derived as (m - M)(0) = 19.93 +/- 0.1 (corresponding to a distance D = 96.8 +/- 4 kpc) from the V-band magnitude of the horizontal branch (V-0,V-HB = 20.45 +/- 0.02). The mean metallicity of the RGB stars is estimated by the V - I colour as [Fe/H] similar to -2.0. The turn-off age estimated by overlaying the theoretical isochrones reveals that most of stars in the UMa I dSph are formed at a very early epoch (similar to 12 Gyr ago). The isopleth map of stellar number density of the UMa I dSph, based upon the resolved star counts of MS, RGB, HB stars as well as blue stragglers (BS), shows that the morphology of the UMa I dSph is quite irregular and distorted, suggesting that the galaxy is in a process of disruption. The very old and metal-poor nature of the stellar population implies that the star formation history of this newly discoverd faint dSph may have been different from other well-known "classical" dSphs, which show significant stellar populations of intermediate age. The stellar population of the UMa I dSph closely resembles that of Galactic old metal-poor globular clusters, but its size is typical of Galactic dSphs (r(e) = 188 [pc], r(1/2) = 300 [pc]), and the shape of its spatial density contours suggests that it is undergoing tidal disruption. These characteristics of stellar population and spatial distribution of the faint galaxies help us to understand how they formed and evolved, and give a hint to the nature of the building blocks of hierarchical galaxy formation.