FORMATION OF ELLIPTIC GALAXIES AT MODERATE REDSHIFTS

Different cosmological models make specific predictions about the numb er of elliptical galaxies as a function of redshift(1), so observation s can in principle be used to discriminate between those models, Tradi tionally, elliptical galaxies have been thought to have formed in a si ngle, rapid burst of star formation at high redshifts (z > 5), and the n evolved quietly-with no significant further star formation-since tha t time(2,3). Yet evidence suggests that at least some ellipticals form ed for the merger of two spiral galaxies(4,5). It remains unclear whic h process dominates the formation of elliptical galaxies. Here I use t he results of deep optical(6) and near-infrared(7-10) images to shaw t hat there are fewer galaxies with very red colours than predicted by m odels 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.