Near-infrared observations of the Fornax dwarf galaxy - I. The red giant branch

Aims. We present a study of the evolved stellar populations in the dwarf spheroidal galaxy Fornax based on wide-area near-infrared observations, aimed at obtaining new independent estimates of its distance and metallicity distribution. Assessing the reliability of near-infrared methods is most important in view of future space- and ground-based deep near-infrared imaging of resolved stellar systems. Methods. We have obtained JHK imaging photometry of the stellar populations in Fornax. The observations cover an 18.5 x 18.5 arcmin(2) central area with a mosaic of SOFI images at the ESO NTT. Our data sample all the red giant branch (RGB) for the whole area. Deeeper observations reaching the red clump of helium-burning stars have also been obtained for a 4.5 x 4.5 arcmin(2) region. Results. Near-infrared photometry led to measurements of the distance to Fornax based on the K-band location of the RGB tip and the red clump. Once corrected for the mean age of the stellar populations in the galaxy, the derived distance modulus is (m - M)(0) = 20.74 +/- 0.11, corresponding to a distance of 141 Kpc, in good agreement with estimates from optical data. We have obtained a photometric estimate of the mean metallicity of red giant stars in Fornax from their (J - K) and ( V - K) colors, using several methods. The effect of the age-metallicity degeneracy on the combined optical-infrared colors is shown to be less important than for optical or infrared colors alone. By taking age effects into account, we have derived a distribution function of global metallicity [M/H] from optical-infrared colors of individual stars. Our photometric Metallicity Distribution Function covers the range - 2.0 < [M/H] < - 0.6, with a main peak at [M/ H] similar or equal to - 0.9 and a long tail of metal-poor stars, and less metal-rich stars than derived by recent spectroscopy. If metallicities from Ca II triplet lines are correct, this result confirms a scenario of enhanced metal enrichment in the last 1 - 4 Gyr.