The Redshift Evolution of Early-Type Galaxies in COSMOS: Do Massive Early-Type Galaxies Form by Dry Mergers?

We study the evolution since z ~ 1 of the rest-frame B luminosity function of ETGs in ~0.7 deg2 in the COSMOS field. In order to identify all progenitors of local ETGs we construct the sample of high-z galaxies using two complementary criteria: (1) a morphological selection based on the Zurich Estimator of Structural Types and (2) a photometric selection based on the galaxy properties in the (U - V)-MV color-magnitude diagram. We furthermore constrain both samples so as to ensure that the selected progenitors of ETGs are compatible with evolving into systems that obey the μB-rhl Kormendy relation. Assuming the luminosity evolution derived from studies of the fundamental plane for high-z ETGs, our analysis shows no evidence for a decrease in the number density of the most massive ETGs out to z ~ 0.7: both the morphologically and the photometrically selected subsamples show no evolution in the number density of bright (~L > 2.5L*) ETGs. Allowing for different star formation histories, and cosmic variance, we estimate a maximum decrease in the number density of massive galaxies at that redshift of ~30%. We observe, however, in both the photometric and morphological samples, a deficit of up to about 2-3 of fainter ETGs over the same cosmic period. Our results argue against a significant contribution of recent dissipationless "dry'' mergers to the formation of the most massive ETGs. We suggest that the mass growth in low-luminosity ETGs can be explained with a conversion from z ~ 0.7 to z = 0 of blue, irregular, and disk galaxies into low- and intermediate-mass "red'' ETGs, possibly also through gas-rich mergers.