On the Stellar Populations in NGC 185 and NGC 205 and the Nuclear Star Cluster in NGC 205 fromHubble Space TelescopeObservations

Abstract

[Abridged] We present a 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 that star formation was probably still active about 4 x 10^8 yr ago. Key abundance-related results are: (1) We identify ancient stars with [Fe/H] <~ -1.5 dex by a well-defined horizontal branch (HB). (2) We find a prominent RGB/ faint-AGB clump/ bump- like feature with the same mean V-band magnitude as the HB, within uncertainties; from a comparison with theory, ancient stars have [Fe/H] ~ -1.5 dex, with a higher abundance level for intermediate-age stars. (3) From colour information we infer that the median [Fe/H] > -1.11 +/- 0.08 dex for ancient stars. For NGC 205, we record (m-M)o = 24.76 +/- 0.1 mag, based on the RGB I-band tip magnitude method. We find that stars were probably still forming less than 3 x 10^8 yr ago, which is compatible with star formation triggered by an interaction with M31. Key abundance-related results are: (1) The RGB/ faint-AGB is significantly skewed to redder values than that of a control field in the outskirts of M31; it probably results from a relatively narrow metallicity and or age range for a significant fraction of the dwarf's stars. (2) For ancient stars we infer from colour information that the median [Fe/H] > -1.06+/-0.04 dex. We briefly compare the stellar populations of NGC 205, NGC 185 and NGC 147. Finally, we find an apparent blue excess in the outer region of the nuclear star cluster in NGC 205. It is as compact as a typical galactic globular cluster, but is quite bright (10^6 L_solar,R); and by matching its blue colour with models, its stellar population is young, up to a few times 10^8 yr old.