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

¹ Department of Astronomy, Yonsei University, Seoul 120-749, Korea e-mail: sohnyj@yonsei.ac.kr;breeze82@galaxy.yonsei.ac.kr
² Center for Space Astrophysics, Yonsei University, Seoul 120-749, Korea
³ Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea
⁴ Astronomy Division, Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea

Received: 23 December 2005
Accepted: 26 January 2006

Abstract

Images of and 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 on the central bar of the dwarf irregular galaxy NGC 6822. The upper envelope of near-infrared and  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 CMD. We select AGB stars in NGC 6822 from the CMD, which has a long spectral wavelength base line in the color index. There are 141 C stars with a mean absolute magnitude of , and with mean colors of , and . The number ratio of C stars to M-giants (C/M) is estimated as , while the northern part of the bar has a somewhat lower C/M ratio than the southern part. The and  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. The M_K 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 . The bolometric LF for M-giants extends up to , while the one for C stars spans .