The AGB population of NGC 6822 : Distribution and the C/M ratio from JHK photometry.

Abstract

Context. NGC 6822 is an irregular dwarf galaxy and part of the Local Group. Its close proximity and apparent isolation provide a unique opportunity to study galactic evolution without any obvious strong external influences. Aims. This paper aims to study the spatial distribution of the asymptotic giant branch (AGB) population and metallicity in NGC 6822. Methods. Using deep, high quality JHK photometry, taken with WFCAM on UKIRT, carbon- and oxygen-rich AGB stars have been isolated. The ratio between their number, the C/M ratio, has then been used to derive the [Fe/H] abundance across the galaxy. Results. The tip of the red giant branch is located at K-0 = 17.41 +/- 0.11 mag and the colour separation between carbon- and oxygen-rich AGB stars is at (J - K)(0) = 1.20 +/- 0.03 mag (i.e. (J - K)(2MASS) similar to 1.28 mag). A C/M ratio of 0.62 +/- 0.03 has been derived in the inner 4 kpc of the galaxy, which translates into an iron abundance of [Fe/H] = -1.29 +/- 0.07 dex. Variations of these parameters were investigated as a function of distance from the galaxy centre and azimuthal angle. Conclusions. The AGB population of NGC 6822 has been detected out to a radius of 4 kpc giving a diameter of 56 arcmin. It is metal-poor, but there is no obvious gradient in metallicity with either radial distance from the centre or azimuthal angle. The detected spread in the TRGB magnitude is consistent with that of a galaxy surrounded by a halo of old stars. The C/M ratio has the potential to be a very useful tool for the determination of metallicity in resolved galaxies but a better calibration of the C/M vs. [Fe/H] relation and a better understanding of the sensitivities of the C/M ratio to stellar selection criteria is first required.