On the Stellar Populations in NGC 185 and NGC 205 and the Nuclear Star Cluster in NGC 205 from Hubble Space Telescope Observations

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 4 × 108 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 3 × 108 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.