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

TL;DR

This study provides detailed photometric analysis of the Ursa Major I dwarf spheroidal galaxy, revealing its old, metal-poor stellar population, irregular morphology, and signs of ongoing tidal disruption, based on wide-field Subaru/Suprime-Cam observations.

Contribution

First deep wide-field photometric study of UMa I dSph revealing its stellar population, structure, and evidence of tidal disruption, with implications for its formation history.

Findings

Distance of 96.8 kpc determined from horizontal branch magnitude

Most stars formed around 12 Gyr ago, indicating an ancient population

Galaxy shows irregular shape and signs of tidal disruption

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 $(m-M)_0=19.93\pm0.1$ (corresponding to a distance D$=96.8\pm4$ kpc) from the V-band magnitude of the horizontal branch (V$_{0,HB}=20.45\pm0.02$). 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 ($\sim12$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.