Near-infrared properties of asymptotic giant branch stars in nearby dwarf elliptical Galaxy NGC 205

Abstract

We investigated the distribution of resolved asymptotic giant branch (AGB) stars over a much larger area than covered by previous near-infrared studies in the nearby dwarf elliptical galaxy NGC 205. Using data obtained with the WIRCam near-infrared imager of the CFHT, we selected the AGB stars in the JHKs color-magnitude diagrams, and separated the C stars from M-giant stars in the JHKs color-color diagram. We identified 1,550 C stars in NGC 205 with a mean absolute magnitude of M_Ks = -7.49 \pm 0.54, and colors of (J - Ks) = 1.81 \pm 0.41 and (H - Ks) = 0.76 \pm 0.24. The ratio of C stars to M-giant stars was estimated to be 0.15 \pm 0.01 in NGC 205, and the local C/M ratios for the southern region are somewhat lower than those for the northern region. The (J - Ks) color distributions of AGB stars contain the main peak of the M-giant stars and the red tail of the C stars. A comparison of the theoretical isochrone models with the observed color distribution indicates that most of the bright M-giant stars in NGC 205 were formed at log(t_yr) \sim 9.0-9.7. The logarithmic slope of the M_Ks luminosity function for M-giant stars was estimated to be 0.84 \pm 0.01, which is comparable with dwarf elliptical galaxies NGC 147 and NGC 185. Furthermore, we found that the logarithmic slopes of the M_Ks luminosity function for C and M-giant stars are different to places, implying a different star formation history within NGC 205. The bolometric luminosity function for M-giant stars extends to M_bol = -6.0 mag, and that for C stars spans -5.6 < M_bol < -3.0. The bolometric luminosity function of C stars is unlikely to be a Gaussian distribution and the mean bolometric magnitude of C stars is estimated to be M_bol = -4.24 \pm 0.55, which is consistent with our results for dwarf elliptical galaxies NGC 147 and NGC 185.