Detection of Strong Evolution in the Population of Early-Type Galaxies

The standard picture holds that giant elliptical galaxies formed in a single burst at high redshift. Aging of their stellar populations subsequently caused them to fade and become redder. The Canada-France Redshift Survey provides a sample of about 150 galaxies with the luminosities and colours of passively evolving giant ellipticals and with 0.1 < z < 1. This sample is inconsistent with the standard evolutionary picture with better than 99.99% confidence. The standard Schmidt test gives 〈V/Vmax〉 = 0.40 when restricted to objects with no detected star formation, and 〈V/Vmax〉 = 0.42 when objects with emission lines are also included. With increasing redshift a larger and larger fraction of the nearby elliptical and S0 population must drop out of of the sample, either because the galaxies are no longer single units or because star formation alters their colours. If the remaining fraction is modelled as F = (1 + z) , the data imply γ = 1.5 ± 0.3. At z = 1 only about one third of bright E and S0 galaxies were already assembled and had the colours of old passively evolving systems. We discuss the sensitivity of these results to the incompleteness corrections and stellar population models we have adopted. We conclude that neither is uncertain enough to reconcile the observations with the standard picture. Hierarchical galaxy formation models suggest that both merging and recent star formation play a role in the strong evolution we have detected.