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 ≈ 6 galaxies in the GOODS-S field studied earlier elsewhere.
Results: We find that the apparent Balmer/4000 Å breaks observed in a number of z ≈ 6 galaxies detected at ⪆ 3.6 μm 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 ≈ 6 galaxies, we find that this effect may lead to a typical downward revision of their stellar ages by a factor ∼ 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.