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

Free access article

Issue		A&A Volume 454, Number 3, August II 2006


Page(s)		717 - 727
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054763

A&A 454, 717-727 (2006)
DOI: 10.1051/0004-6361:20054763

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

A. Kang^{1, 2}, Y.-J. Sohn^{1, 2}, H.-I. Kim³, J. Rhee^{1, 2}, J.-W. Kim^{1, 2}, N. Hwang⁴, M. G. Lee⁴, Y.-C. Kim¹ and M.-S. Chun¹

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 ${\it JHK}$ and ${\it gi}$ 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 $3\farcm6\times6\farcm3$ 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 $\langle M_{K}\rangle=-7.60\pm0.50$ , and with mean colors of $\langle (J-K)_{0}\rangle=1.77\pm0.41$ , and $\langle (H-K)_{0}\rangle=0.75\pm0.28$ . The number ratio of C stars to M-giants (C/M) is estimated as $0.27\pm0.03$ , 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 $(t_{\rm yr})\sim9.0$ . 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 $\langle M_{\rm bol}\rangle=-4.36\pm0.54$ . The bolometric LF for M-giants extends up to $M_{\rm bol}=-6.5$ , while the one for C stars spans $-5.8<M_{\rm bol}<-3.5$ .

Key words: galaxies: individual: NGC 6822 -- galaxies: stellar content -- stars: AGB and post-AGB

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