THE SHAPES AND AGES OF ELLIPTIC GALAXIES

In this paper we investigate the relation between the detailed isophot al shape of elliptical galaxies and the strength of the H beta absorpt ion in their spectra. We find that discy galaxies have higher H beta i ndices. Stellar population synthesis models shaw that the H beta line is a good age indicator, hence discy galaxies tend to have lower mean ages than boxy galaxies. We show that the observed trend can be brough t about by a contaminating young population, which we associate with t he discy component. This population need only account for a small frac tion of the total mass: for example, if a contaminating population of age 2 Gyr is superimposed on an old (12 Gyr) elliptical galaxy, then t he observed trend can be explained if it contributes only 10 per cent to the total mass. The size of this effect is consistent with the esti mates of disc-to-total light ratios from surface photometry.