Probing the evolution of early-type galaxies using multicolour number counts and redshift distributions
Abstract
We investigate pure luminosity evolution models for early-type (elliptical and S0) galaxies (i.e. no number density change or morphological transition), and examine whether these models are consistent with observed number counts in the B, I and K bands, and redshift distributions of two samples of faint galaxies selected in the I and K bands. The models are characterized by the star formation time-scale τSF and the time tgw when the galactic wind starts to blow, in addition to several other conventional parameters. We find that the single-burst model (τSF=0.1 Gyr and tgw=0.353 Gyr), which is known to reproduce the photometric properties of early-type galaxies in clusters, is inconsistent with the redshift distributions of early-type galaxies in the field environment, owing to overpredictions of the number of galaxies atz≳1.4 even with strong extinction which is at work until tgw. In order for dust extinction to be more effective, we treat τSF and tgw as free parameters, and find that models with τSF≳0.5 Gyr and tgw>1.0 Gyr can be made consistent with both the observed redshift distributions and the number counts, if we introduce strong extinction [E(B−V)≥1 as a peak value]. These results suggest that early-type galaxies in the field environment do not have the same evolutionary history as described by the single-burst model.
