Near-infrared observations of the Fornax dwarf galaxy ^*,**

I. The red giant branch

¹ Osservatorio Astronomico di Padova, INAF, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: marco.gullieuszik@oapd.inaf.it
² Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, 35122 Padova, Italy
³ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 22 November 2006
Accepted: 6 March 2007

Abstract

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 18.518.5 arcmin² 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 4.54.5 arcmin² 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    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 and 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]-0.9 and a long tail of metal-poor stars, and less metal-rich stars than derived by recent spectroscopy. If metallicities from  triplet lines are correct, this result confirms a scenario of enhanced metal enrichment in the last 1–4 Gyr.