1999A&A...345..747H

We have obtained deep CCD photometry in the B, V, and I bands of Phoenix, a galaxy considered a transition case between dwarf spheroidal (dSph) and dwarf irregular (dI) galaxies. A comparison of our data with the giant branches of Galactic globular clusters gives a mean metal abundance [Fe/H]=-1.81±0.10dex. The presence of an intrinsic color dispersion in the upper red giant branch (RGB) suggests an abundance range of about 0.5dex, although a range in age may also affect the RGB width. The color-magnitude diagram (cmd) of Phoenix reveals for the first time a horizontal branch (HB) predominantly red yet moderately extended to the blue, similar to those of Leo II or And I, at V≃23.8. The detection of a relatively blue HB indicates the presence of a significant population with age comparable to that of old halo Galactic globular clusters. As in other dwarf spheroidals, this HB morphology in a metal-poor system indicates a mild ``second parameter'' effect. The mean level of the HB has been used to derive a true distance modulus 23.21±0.08, in good agreement with the distance modulus 23.04±0.07 estimated from the well defined cutoff of the red giant branch at I≃23.1. This confirms the correct identification of the RGB tip. We also find a radial gradient in the Phoenix HB morphology, as measured by an increasing ratio of blue HB stars to red giant stars in the outskirts of the galaxy. The color-magnitude diagrams show a small number of stars above the tip of the RGB, well in excess over field contamination, that most likely are asymptotic giant branch (AGB) stars belonging to an intermediate age population. Their number indicates that the fraction of intermediate age (3 to 10Gyr) 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 0.6Gyr ago, up to 1x10⁸yr 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, including an extended star formation history and even the presence of a modest amount of neutral hydrogen, Phoenix appears not dissimilar from dwarf spheroidal galaxies in the Local Group.