The Absence of Passively Evolving Ellipticals in Deep Optical and Near-Infrared Surveys
The traditional view of elliptical galaxies has been that they formed in a single, rapid burst of star formation at high redshift, and have evolved quiescently since that time. In opposition to this traditional view is evidence that at least some elliptical galaxies have formed from the merger of two disk galaxies. What has not been clear is which process is the dominant formation mechanism for the large majority of elliptical galaxies. This question has significant implications for cosmological models, as different models make different predictions for the formation mechanism and epoch of elliptical galaxies. Here I use deep optical and near-infrared images to show that there are fewer galaxies with very red optical and near-infrared colors than predicted by models in which typical elliptical galaxies have completed their star formation by z >= 5. These observations require that elliptical galaxies have significant star formation at z < 5. This requirement, combined with constraints on lower redshift starbursts from the modest ultraviolet luminosities of galaxies in the Hubble Deep Field and the properties of galaxies from 0 < z < 1, suggests that elliptical galaxies form at moderate redshift in dusty starbursts and/or through the hierarchical merging of smaller objects.