The impact of nebular emission on the ages of z ≈ 6 galaxies

Aims. We examine the influence of nebular continuous and line emission in high-redshift star forming galaxies on determinations of their age, formation redshift, and other properties from SED fits. Methods. We include nebular emission consistently with the stellar emission in our SED fitting tool and analyse differentially a sample of 10 z approximate to 6 galaxies in the GOODS-S field studied earlier elsewhere. Results. We find that the apparent Balmer/4000 angstrom breaks observed in a number of z approximate to 6 galaxies detected at >= 3.6 mu m with IRAC/Spitzer can be mimicked by the presence of strong restframe optical emission lines, implying in particular younger ages than previously thought. Applying these models to the small sample of z approximate to 6 galaxies, we find that this effect may lead to a typical downward revision of their stellar ages by a factor similar to 3. In consequence their average formation redshift may be reduced drastically, and these objects may not have contributed to cosmic reionisation at z > 6. Extinction and stellar mass estimates may also be somewhat modified, but to a lesser extent. Conclusions. Careful SED fits including nebular emission and treating properly uncertainties and degeneracies are necessary for more accurate determinations of the physical parameters of high-z galaxies.