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

TL;DR

This study shows that nebular emission significantly affects the estimated ages and formation redshifts of z~6 galaxies, suggesting they are younger than previously thought and may not have contributed to reionization.

Contribution

It introduces a method to include nebular emission in SED fitting, revealing that galaxy ages at high redshift are often overestimated without this correction.

Findings

Nebular emission can mimic Balmer breaks, leading to younger age estimates.

Inclusion of nebular emission reduces estimated galaxy ages by a factor of about 3.

High-redshift galaxy formation redshifts may be lower, affecting reionization contributions.

Abstract

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. 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 by Eyles et al. (2007). We find that the apparent Balmer/4000 Ang breaks observed in a number of z~6 galaxies detected at >~3.6 micron 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 drastically be reduced, 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. 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.