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