Stellar populations in the Phoenix dwarf galaxy

Abstract

We have obtained deep BVI CCD photometry of Phoenix, a galaxy considered a transition case between dwarf spheroidal and dwarf irregular galaxies. A comparison of our data with the RGBs of Galactic globular clusters gives a mean metal abundance [Fe/H] = -1.81+/-0.10 dex. The presence of an intrinsic color dispersion in the upper red RGB suggests an abundance range of about 0.5 dex, although a range in age may also affect the RGB width. For the first time, a HB has been revealed at V~23.8. The HB is predominantly red yet moderately extended to the blue, which indicates the presence of a significant population with age comparable to that of old halo GGCs. This HB morphology in a metal-poor system indicates a mild "second parameter" effect. From the mean level of the HB we derived a true distance modulus 23.21+/-0.08, in good agreement with the distance modulus 23.04+/-0.07 estimated from the cutoff of the RGB at I~23.1. We find a radial gradient in the HB morphology, and our CMDs show a small number of stars above the RGB tip, that most likely are AGB stars of an intermediate age population. Their number indicates that the fraction of intermediate age population in Phoenix is approximately 30-40%. A young stellar population is definitely present in Phoenix, consistent with a star formation episode started at least 0.6 Gyr ago, up to 1x10^{8} yr ago. Both young stars and AGB stars are centrally concentrated, which indicates that recent star formation preferentially occurred in the inner galaxy regions. In many respects, Phoenix appears not dissimilar from dwarf spheroidal galaxies in the Local Group.