2005AJ....129.2217B

We present the first detailed analysis of resolved stellar populations in the dwarf galaxies NGC 185 and NGC 205 based on archival V- and I-band WFPC2 pointings. For NGC 185 we deduce from the brightest main-sequence and blue-loop stars that star formation was probably still active about 4x10⁸ yr ago and have three key abundance-related results: (1) We identify ancient stars with [Fe/H]≲-1.5 dex by a well-defined horizontal branch (HB). (2) We find a prominent clump/bump-like feature along the red giant branch/faint asymptotic giant branch (RGB/faint AGB) with the same mean V-band magnitude as in the HB, within uncertainties [i.e., ΔV(bump-HB)=0]; from a comparison with theory, the implication is that ancient stars have [Fe/H]~-1.5 dex, with a higher abundance level for intermediate-age stars. (3) From color information we infer that the median [Fe/H]>-1.11±0.08 dex for ancient stars [assuming E(B-V)=0.18 mag]. For NGC 205 we record a new distance modulus, (m-M)₀=24.76±0.1 mag, taking E(B-V)=0.11 mag, based on the RGB tip magnitude method in the I band. We find that stars were probably still forming less than 3x10⁸ yr ago in NGC 205, which is compatible with star formation triggered by an interaction with M31. There are three key abundance-related results for NGC 205: (1) The RGB/faint AGB is significantly skewed to redder values than those for a control field in the outskirts of M31; this probably results from a relatively narrow metallicity and/or age range for a significant fraction of the dwarf's stars. (2) From a comparison with models, the most metal-rich RGB stars reach [Fe/H]≳-0.7 dex (≳0.2 Z_☉). (3) For ancient stars we infer from color information that the median [Fe/H]>-1.06±0.04 dex [for E(B-V)=0.11 mag]. We briefly compare the stellar populations of NGC 205, NGC 185, and NGC 147. Finally, we study several V- and R-band structural properties of the nuclear star cluster in NGC 205 for the first time; the apparent V- and R-band effective radii indicate a blue excess in the cluster's outer region. In terms of size, the cluster is like a typical Galactic globular cluster or a nuclear cluster in a nearby late-type spiral galaxy, but it is quite bright (10⁶ L_☉,R_), unlike an ancient globular cluster, and by matching with models, the blue color hints that its stellar population is young, up to a few times 10⁸ yr old.