Probing early-type galaxy evolution with the Kormendy relation

We investigate the evolution of early-type galaxies in four clusters at z=0.4 (Abell370, Cl0303+17, Cl0939+47 and Cl1447+26) and in one at z=0.55 (Cl0016+16). The galaxies are selected according to their spectrophotometrically determined spectral types and comprise the morphological classes E, S0 and Sa galaxies. Structural parameters are determined by a two-component fitting of the surface brightness profiles derived from HST images. Exploring a realistic range of K-corrections using Bruzual and Charlot models, we construct the rest-frame B-band Kormendy relations (mu_e-log(R_e)) for the different clusters. We do not detect a systematic change of the slope of the relation as a function of redshift. We discuss in detail how the luminosity evolution, derived by comparing the Kormendy relations of the distant clusters with the local one for Coma, depends on various assumptions and give a full description of random and systematic errors by exploring the influences of selection bias, different star formation histories and K-corrections. Early-type galaxies with modest disk components (S0 and Sa) do not differ significantly in their evolution from disk-less ellipticals. The observed luminosity evolution is compatible with pure passive evolution models (with redshift of formation z>2) but also with models that allow ongoing star formation on a low level, like exponentially decaying star formation models with an e-folding time of tau=1 Gyr.