  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.
