On the interpretation of the age and chemical composition of composite stellar populations determined with line-strength indices

We study the simple stellar population-equivalent (SSP-equivalent) age and chemical composition measured from the Lick/IDS line-strength indices of composite stellar populations (CSPs). We build two sets of similar to 30 000 CSP models using stellar populations synthesis models, combining an old population and a young population with a range of ages and chemical compositions representative of early-type galaxies. We investigate how the SSP-equivalent stellar parameters of the CSPs depend on the stellar parameters of the two input populations; how they depend on V-band luminosity-weighted stellar parameters; and how SSP-equivalent parameters derived from using different Balmer-line indices can be used to reveal the presence of a young population on top of an old one. We find that the SSP-equivalent age depends primarily on the age of the young population and on the mass fraction of the two populations, and that the SSP-equivalent chemical composition depends mainly on the chemical composition of the old population. Furthermore, the SSP-equivalent chemical composition tracks quite closely the V-band luminosity-weighted one, while the SSP-equivalent age is strongly biased towards the age of the young population. In this bias, the age of the young population and the mass fraction between old and young population are degenerate. Finally, assuming typical error bars, we find that a discrepancy between the SSP-equivalent parameters determined with different Balmer-line indices can reveal the presence of a young stellar population on top of an old one as long as the age of the young population is less than similar to 2.5 Gyr and the mass fraction of young-to-old population is between 1 and 10 per cent.