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 \pm 0.11 mag and the colour separation between carbon- and oxygen-rich AGB stars is at ( J - K ) _ { 0 } = 1.20 \pm 0.03 mag ( i.e . ( J - K ) _ { 2 MASS } \sim 1.28 mag ) .
A C/M ratio of 0.62 \pm 0.03 has been derived in the inner 4 kpc of the galaxy , which translates into an iron abundance of [ Fe/H ] = -1.29 \pm 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 . Tables .
2 , 3 and 4 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/