The shapes and ages of elliptical galaxies

In this paper we investigate the relation between the detailed isophotal shape of elliptical galaxies and the strength of the H beta absorption in their spectra. We find that discy galaxies have higher H beta indices. 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 brought about by a contaminating young population, which we associate with the discy component. This population need only account for a small fraction of the total mass: for example, if a contaminating population of age 2 Gyr is superimposed on an old (12 Gyr) elliptical galaxy, then the 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 estimates of disc-to-total light ratios from surface photometry.