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

Abstract

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\arcmin $\times$ 27\arcmin 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.

6 pages, 6 figures. Accepted for publication in A&A

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