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Date and Time of the Query: 2019-06-19 T02:09:22 PDT
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For refcode 2005AJ....129.2217B:
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2005AJ....129.2217B On the Stellar Populations in NGC 185 and NGC 205 and the Nuclear Star Cluster in NGC 205 from Hubble Space Telescope Observations D. J. Butler and D. Martinez-Delgado Abstract. 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 4x10^8^ 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., {DELTA}V(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 3x10^8^ 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_sun_). (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 (10^6^ L_sun,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 10^8^ yr old. Keywords: Galaxies: Dwarf, Galaxies: Evolution, Galaxies: Individual: NGC 147, Galaxies: Individual: NGC 185, Galaxies: Individual: NGC 205, Galaxies: Photometry, Galaxies: Local Group
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