Detection of strong evolution in the population of early-type galaxies

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Abstract
The standard picture holds that giant elliptical galaxies formed in a single burst at high redshift. Ageing of their stellar populations subsequently caused them to fade and become redder. The Canada-France Redshift Survey provides a sample of about 125 galaxies with the luminosities and colours of passively evolving early-type galaxies and with 0.1< z < 1. This sample is inconsistent with the standard evolutionary picture for elliptical galaxies with better than 99.9 per cent confidence. The standard Schmidt test gives 〈V/Vmax〉 = 0.398 when restricted to objects with no detected star formation, and 〈V/Vmax〉 = 0.410 when objects with emission lines are also included. A smaller sample of early-type galaxies selected from the Hawaii Deep Survey gives equally significant results. With increasing redshift, a larger and larger fraction of the nearby elliptical and S0 population must drop out 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) −γ, the data imply that γ = 1.5 ± 0.4. At z = 1, only about one-third of nearby 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 that we have adopted. We conclude that neither the corrections nor the models are 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 that we have detected.

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