The metallicity of the red giant branch in the disk of NGC 6822

Deep J, H, and K' images obtained with the Canada-France-Hawaii Telescope adaptive optics system are used to investigate the metallicity of red giant branch (RGB) stars in three fields in the disk of the Local Group dwarf irregular galaxy NGC 6822. The slope of the RGB on the (K, J-K) color-magnitude diagrams indicates that ([Fe/H]) over bar = -1.0 +/- 0.3. The locus of the RGB is bluer than that of globular clusters with the same RGB slope by an amount that is consistent with the majority of RGB stars in these fields having an age near 3 Gyr. It is demonstrated that if RGB stars in NGC 6822 are this young, then the metallicity computed from the RGB slope may be similar to0.05 dex too low. Finally, the metallicity computed from the RGB slope is lower than spectroscopic-based metallicities for young stars in NGC 6822, and it is concluded that Delta[Fe/H]/Deltat = -0.2 +/- 0.1 dex Gyr(-1) in NGC 6822 during the past few gigayears.