THE ASYMPTOTIC GIANT BRANCH OF NGC 205: THE CHARACTERISTICS OF CARBON STARS AND M GIANTS IDENTIFIED FROM JHK′ IMAGES

摘要

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′ 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 occurred 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. The techniques used here to identify C stars and probe the AGB are well suited to studies of galaxies outside of the Local Group using data obtained with adaptive optics systems on large ground-based telescopes.