On the stellar populations in NGC 185 and NGC 205 and the nuclear star cluster in NGC 205 from Hubble Space Telescope observations

被引:51
作者
Butler, DJ [1 ]
Martínez-Delgado, D [1 ]
机构
[1] Max Planck Inst Astron, D-69117 Heidelberg, Germany
关键词
galaxies : dwarf; galaxies : evolution; galaxies; individual; (NGC; 147; NGC; 185; 205); galaxies : photometry; Local Group;
D O I
10.1086/429524
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
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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收藏
页码:2217 / 2231
页数:15
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