Probing the evolution of early-type galaxies using multicolour number counts and redshift distributions

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 cou nts in the B, I and K bands, and redshift distributions of two samples of f aint galaxies selected in the I and K bands. The models are characterized b y the star formation time-scale tau(SF) and the time t(gw) when the galacti c wind starts to blow, in addition to several other conventional parameters . We iind that the single-burst model (tau(SF) = 0.1Gyr and t(gw) = 0.353Gy r), which is known to reproduce the photometric properties of early-type ga laxies in clusters, is inconsistent with the redshift distributions of earl y-type galaxies in the field environment, owing to overpredictions of the n umber of galaxies at z greater than or similar to 1.4 even with strong exti nction which is at work until t(gw). In order for dust extinction to be mor e effective, we treat tau(SF) and t(gw) as free parameters, and find that m odels with tau(SF) greater than or similar to 0.5 Gyr and t(gw) > 1.0 Gyr c an be made consistent with both the observed redshift distributions and the number counts, if we introduce strong extinction [E(B - V) greater than or equal to 1 as a peak value]. These results suggest that early-type galaxie s in the field environment do not have the same evolutionary history as des cribed by the single-burst model.