The Asymptotic Giant Branch of NGC 205: The Characteristics of Carbon Stars and M Giants Identified from JHK' Images

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Abstract
J, H, and K' images are used to investigate the asymptotic giant branch (AGB) content of the Local Group dwarf elliptical galaxy NGC 205. The AGB on the (K, H-K) and (K, J-K) color-magnitude diagrams consists of two sequences: a near-vertical plume of giants with spectral types K and M, and a red arm containing C stars. There are 320 C stars with M_bol < -4.1 and J-K > 1.5 within 2 arcmin of the nucleus. C stars account for 10% of the integrated luminosity of AGB stars brighter than M_bol = -3.75 near the center of NGC 205, and this is in excellent agreement with what is measured in intermediate-age clusters in the LMC. The most luminous AGB star has M_bol = -6.5, although variability introduces an uncertainty of a few tenths of a magnitude when using this as an estimate of the AGB-tip brightness. Comparisons with models suggest that the brightest AGB stars formed within the past 0.1 Gyr, and that the previous episode of star formation occured a few tenths of a Gyr earlier. These results are consistent with star formation in NGC 205 being triggered by interactions with M31. These data also demonstrate that near-infrared imaging provides an efficient means of identifying C stars in nearby galaxies.

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