Journal
ASTRONOMICAL JOURNAL
Volume 129, Issue 5, Pages 2217-2231Publisher
IOP PUBLISHING LTD
DOI: 10.1086/429524
Keywords
galaxies : dwarf; galaxies : evolution; galaxies : individual (NGC 147, NGC 185, NGC 205); galaxies : photometry; Local Group
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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 x 10(8) yr ago and have three key abundance-related results: ( 1) We identify ancient stars with [Fe/H] less than or similar to - 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] similar to -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] greater than or similar to - 0.7 dex (greater than or similar to 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 (10(6) 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 10(8) yr old.
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