We have obtained deep CCD photometry in the B , V , and I Â 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 \pm 0.10 Â dex .
The presence of an intrinsic color dispersion in the upper red giant branch ( RGB ) suggests an abundance range of about 0.5 dex , 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 \approx 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 \pm 0.08 , in good agreement with the distance modulus 23.04 \pm 0.07 Â estimated from the well defined cutoff of the red giant branch at I \approx 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 10 Gyr ) 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 1 \times 10 ^ { 8 } Â 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 , 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 .