We present deep and wide V , I CCD photometry of Ursa Major I ( UMa I ) dwarf spheroidal galaxy ( dSph ) in Local Group .
The images of the galaxy were taken by Subaru/Suprime-Cam wide field camera , covering a field of 34′ \times 27′ located at the centre of the galaxy .
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 ,MSTO } \sim 23.5 mag .
The distance modulus is derived as ( m - M ) _ { 0 } = 19.93 \pm 0.1 ( corresponding to a distance D = 96.8 \pm 4 kpc ) from the V-band magnitude of the horizontal branch ( V _ { 0 ,HB } = 20.45 \pm 0.02 ) .
The mean metallicity of the RGB stars is estimated by the V - I colour to [ Fe/H ] \sim - 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 very early epoch ( \sim 12 Gyrs 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 population of intermediate age .
The stellar population of the UMa I dSph closely resembles that of Galactic old metal-poor globular cluster , 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 know how they formed and evolved , and give a hint to the nature of the building blocks of hierarchical galaxy formation .