The age of elliptical galaxies and bulges in a merger model

The tightness of the observed colour-magnitude relation for elliptical galaxies has often been cited as an argument against a model in which ellipticals form by the merging of spiral discs. A common view is that merging would mix together stars of disparate ages and produce a large scatter in colour. Here we use semi-analytic models of galaxy formation in a flat cold dark matter (CDM) universe to derive the distribution of the mean ages and colours of the stars in cluster elliptical galaxies formed by mergers. It is seen that most of the stars in cluster ellipticals form at relatively high redshift (z>1.9) and that the predicted scatter in the colour-magnitude relation falls within observational bounds. We conclude that the apparent 'homogeneity' in the properties of the stellar populations of cluster ellipticals is not inconsistent with a merger scenario for the origin of these systems. The analysis is then extended to the stellar populations of field ellipticals and the bulges of spiral galaxies. The mean stellar age of a bright field elliptical is on average 4 Gyr less than that of a cluster elliptical. The ages of bulges are found to correlate strongly with the luminosity of the associated disc. The bulges of late-type spirals are predicted to be older than the bulges of early-type spirals. Finally, we derive predictions for the properties of elliptical galaxies in clusters at high redshift. The number of recently merged objects increases with redshift and this causes the scatter in the colour distributions to become progressively larger. However, an rms scatter in rest-frame U - V colour greater than 0.1 is predicted only at redshifts greater than 1.