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

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 K₀ = 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 ~ 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.

Full Tables 2-4 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/540/A135