Volume 467, Number 3, June I 2007
|Page(s)||1025 - 1036|
|Published online||13 March 2007|
I. The red giant branch
Osservatorio Astronomico di Padova, INAF, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: email@example.com
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
Accepted: 6 March 2007
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.
Key words: galaxies: dwarf / galaxies: individual: Fornax / Local Group / galaxies: stellar content
Based on data collected at the European Southern Observatory, La Silla, Chile, Proposals No. 65.N-0167, 66.B-0247.
© ESO, 2007
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.