1999MNRAS.309L..25N

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 τ_SF and the time t_gw when the galactic wind starts to blow, in addition to several other conventional parameters. We find that the single-burst model (τ_SF=0.1Gyr and t_gw=0.353Gyr), 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 at z≳1.4 even with strong extinction which is at work until t_gw. In order for dust extinction to be more effective, we treat τ_SF and t_gw as free parameters, and find that models with τ_SF≳0.5Gyr and t_gw>1.0Gyr 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.