Stellar populations in the Phoenix dwarf galaxy

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 dw arf irregular (dI) galaxies. A comparison of our data with the giant branch es 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 r ed giant branch (RGB) suggests an abundance range of about 0.5 dex, althoug h 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) predo minantly red yet moderately extended to the blue, similar to those of Leo I I or And I, at V approximate to 23.8. The detection of a relatively blue HE indicates the presence of a significant population with age comparable to that of old halo Galactic globular clusters. As in other dwarf spheroidals, this HE morphology in a metal-poor system indicates a mild "second paramet er" effect. The mean level of the HE has been used to derive a true distanc e 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 approximate to 23.1. This confirms the correct identification of the RGB tip. We also find a radial gradient in the Phoenix HE morphology, as measu red by an increasing ratio of blue HE stars to red giant stars in the outsk irts of the galaxy. The color-magnitude diagrams show a small number of sta rs above the tip of the RGB, well in excess over field contamination, that most likely are asymptotic giant branch (AGB) stars belonging to an interme diate age population. Their number indicates that the fraction of intermedi ate age (3 to 10 Gyr) population in Phoenix is approximately 30-40%. A youn g stellar population is definitely present in Phoenix, consistent with a st ar formation episode started at least 0.6 Gyr ago, up to 1 x 10(8) yr ago. Both young stars and AGE stars are centrally concentrated, which indicates that recent star formation preferentially occurred in the inner galaxy regi ons. In many respects, including an extended star formation history and eve n the presence of a modest amount of neutral hydrogen, Phoenix appears not dissimilar from dwarf spheroidal galaxies in the Local Group.