The evolved asymptotic giant branch stars in the central bar of the dwarf irregular galaxy NGC 6822

Abstract

Images of JHK and gi obtained with the Canada-France-Hawaii Telescope are used to investigate the stellar contents of the resolved asymptotic giant branch (AGB) population in the area of 3′6×6′3 on the central bar of the dwarf irregular galaxy NGC 6822. The upper envelope of near-infrared (J - K, K) and (H - K, K) color-magnitude diagrams (CMDs) shows four components of main-sequence stars, foreground stars, massive supergiants, and AGB stars. These components are also noticeable in the bright part of (g - i, g) CMD. We select AGB stars in NGC 6822 from the (g - K, g) CMD, which has a long spectral wavelength base line in the color index. There are 141 C stars with a mean absolute magnitude of 〈MK〉 = -7.60 ± 0.50, and with mean colors of 〈(J - K)0〉 = 1.77 ± 0.41, and 〈(H - K)0〉 = 0.75 ± 0.28. The number ratio of C stars to M-giants (C/M) is estimated as 0.27 ± 0.03, while the northern part of the bar has a somewhat lower C/M ratio than the southern part. The (J - K) and (H - K) color distributions of AGB stars contain a main peak of M-giant stars and a red tail of C stars. The broad color distributions for M-giants indicate that NGC 6822 has a wide range of intermediate ages with a peak at log(tyr) ∼ 9.0. The MK luminosity function (LF) for C stars in the central bar of NGC 6822 is likely to be a Gaussian distribution, while the LF of C stars in the southern part is slightly skewed to the brighter side than in northern part. The mean bolometric magnitude of 141 C stars in NGC 6822 is 〈Mbol〉 = -4.36 ± 0.54. The bolometric LF for M-giants extends up to Mbol = -6.5, while the one for C stars spans -5.8 < Mbol < -3.5. © ESO 2006.