Formation of elliptical galaxies at moderate redshifts

Different cosmological models make specific predictions about the number of elliptical galaxies as a function of redshift(1), so observations can in principle be used to discriminate between those models, Traditionally, elliptical galaxies have been thought to have formed in a single, rapid burst of star formation at high redshifts (z > 5), and then evolved quietly-with no significant further star formation-since that time(2,3). Yet evidence suggests that at least some ellipticals formed for the merger of two spiral galaxies(4,5). It remains unclear which process dominates the formation of elliptical galaxies. Here I use the results of deep optical(6) and near-infrared(7-10) images to shaw that there are fewer galaxies with very red colours than predicted by models in which typical ellipticals have completed their star formation by z approximate to 5, which means that elliptical galaxies must have significant star formation at z < 5. This requirement, combined with constraints on bursts of star formation in lower-redshift galaxies(11,12), and the observed properties of galaxies in the redshift range 0 < z < 1 (refs 13-17), suggests either that ellipticals form at moderate redshifts, where a large initial burst of star formation is shrouded by dust, or that they form through the merging of smaller galaxies.