2006A&A...445...69S

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 <M_K≥-7.56, <(J-K)₀≥1.81 and <(H-K)₀≥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'') and 0.19±0.03 for the outer region (r>70'') indicate a weak radial gradient. The mean bolometric magnitude of 91 C stars in NGC 147 is <M_bol≥-4.32±0.49. The bolometric luminosity function of M giant stars in NGC 147 extends up to M_bol=-5.8mag, 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 ∼2Gyr.