Near-IR photometry of asymptotic giant branch stars in the dwarf elliptical galaxy NGC 147

Abstract

Near-infrared J, H and K' images were used to investigate the stellar contents of the asymptotic giant branch (AGB) population in the nearby dwarf elliptical galaxy NGC 147. The obtained (K, J - K) and (K, H - K) color-magnitude diagrams contain stars of AGB and red giant branch populations, where the former consists of a group of bright blue stars, a dominant population of M giant and a red C star population. We identified 91 AGB C stars in NGC 147 with the mean absolute magnitude and colors of = -7.56, = 1.81 and = 0.74. The estimated number ratio of C stars to M giant stars (C/M) is 0.16 ± 0.02. The estimated local C/M ratios of 0.14 ± 0.02 for the inner region (r 70) indicate a weak radial gradient. The mean bolometric magnitude of 91 C stars in NGC 147 is = -4.32 ± 0.49. The bolometric luminosity function of M giant stars in NGC 147 extends up to M bol = -5.8 mag, and that of only C stars spans -5.6 < M bol < -3.5. The color histograms of AGB stars in (J - K) and (H - K) show a main peak containing M giant stars, a red tail containing C stars with a weak excess, and a blue excess possibly due to bright blue foreground stars and AGB stars younger than M giants. The comparison of the theoretical isochrone models with the color distribution and the brightness of AGB stars indicates that most of the bright M giants in NGC 147 were formed at log(t yr ) ∼ 8.4 with a range of 8.2 ∼ 8.6. Ages of the majority of M giant stars in NGC 147 are similar to those of younger M giant stars in its companion NGC 185, while their formation occurred more recently than the older M giant stars in NGC 185 by ∼2 Gyr.