Higher-order Balmer line indices in alpha/Fe-enhanced stellar population models

Abstract

We have computed the higher-order Balmer absorption line indices Hgamma and Hdelta (Worthey & Ottaviani 1997) for stellar population models with variable element ratios. The response of these indices to abundance ratio variations is taken from detailed line formation and model atmosphere calculations. We find that Hgamma and Hdelta, unlike Hbeta, are very sensitive to alpha/Fe ratio changes at super-solar metallicities. Both line indices increase significantly with increasing alpha/Fe ratio. This effect cannot be neglected when these line indices are used to derive the ages of metal-rich, unresolved stellar populations like early-type galaxies. We re-analyze the elliptical galaxy sample of Kuntschner (2000), and show that consistent age estimates from Hbeta and Hgamma are obtained, only if the effect of alpha/Fe enhancement on Hgamma is taken into account in the models. This result rectifies a problem currently present in the literature, namely that Hgamma and Hdelta have up to now led to significantly younger ages for early-type galaxies than Hbeta. Our work particularly impacts on the interpretation of intermediate to high-redshift data, for which only the higher-order Balmer lines are accessible observationally.