THE AGE OF ELLIPTIC 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 tha t merging would mix together stars of disparate ages and produce a lar ge scatter in colour. Here we use semi-analytic models of galaxy forma tion in a flat cold dark matter (CDM) universe to derive the distribut ion of the mean ages and colours of the stars in cluster elliptical ga laxies 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 pre dicted scatter in the colour-magnitude relation falls within observati onal bounds. We conclude that the apparent 'homogeneity' in the proper ties of the stellar populations of cluster ellipticals is not inconsis tent with a merger scenario for the origin of these systems. The analy sis is then extended to the stellar populations of field ellipticals a nd the bulges of spiral galaxies. The mean stellar age of a bright fie ld elliptical is on average 4 Gyr less than that of a cluster elliptic al. The ages of bulges are found to correlate strongly with the lumino sity of the associated disc. The bulges of late-type spirals are predi cted to be older than the bulges of early-type spirals. Finally, we de rive predictions for the properties of elliptical galaxies in clusters at high redshift. The number of recently merged objects increases wit h 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.