The Redshift Evolution of Early-Type Galaxies in COSMOS: Do Massive Early-Type Galaxies Form by Dry Mergers?
Kneib, J. -P.; Aussel, H.; Cimatti, A.; Daddi, E.; Sanders, D. B.; Murayama, T.; Taniguchi, Y.; Sargent, M. T.; Halliday, C.; Capak, P.; Carollo, C. M.; McCracken, H. J.; Scoville, N.; Scarlata, C.; Koekemoer, A.; Tasca, L.; Thompson, D.; Mobasher, B.; Massey, R.; Rhodes, J.; Leauthaud, A.; Lilly, S. J.; Renzini, A.; Kampczyk, P.; Takahashi, M.; Shioya, Y.; Sasaki, S.; Feldmann, R.; Ajiki, M.
Switzerland, Italy, United States, France, Japan
Abstract
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.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.