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

We have computed the higher-order Balmer absorption line indices Hγ and Hδ 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 Hγ and Hδ, unlike Hβ, are very sensitive to α/Fe ratio changes at super-solar metallicities. Both line indices increase significantly with increasing α/Fe ratio. This effect cannot be neglected when these line indices are used to derive the ages of metal-rich, unresolved stellar populations such as early-type galaxies. We re-analyze the elliptical galaxy sample of Kuntschner, and show that consistent age estimates from Hβ and Hγ are obtained only if the effect of α/Fe enhancement on Hγ is taken into account in the models. This result rectifies a problem currently present in the literature, namely that Hγ and Hδ have up to now led to significantly younger ages for early-type galaxies than Hβ. Our work particularly impacts on the interpretation of intermediate- to high-redshift data, for which only the higher-order Balmer lines are accessible observationally.