The buildup of stellar mass and the 3.6 μm luminosity function in clusters from z=1.25 to z=0.2

We have measured the 3.6 mu m luminosity evolution of about 1000 galaxies in 32 clusters at 0.2 < z < 1.25, without any a priori assumption about luminosity evolution, i.e. in a logically rigorous way. We find that the luminosity of our galaxies evolves as an old and passively evolving population formed at high redshift without any need for additional redshift-dependent evolution. Models with a prolonged stellar mass growth are rejected by the data with high confidence. The data also reject models in which the age of the stars is the same at all redshifts. Similarly, the characteristic stellar mass evolves, in the last two thirds of the universe age, as expected for a stellar population formed at high redshift. Together with the old age of stellar populations derived from fundamental plane studies, our data seems to suggest that early-type cluster galaxies have been completely assembled at high redshift, and not only that their stars are old. The quality of the data allows us to derive the LF and mass evolution homogeneously over the whole redshift range, using a single estimator. The Schechter function describes the galaxy luminosity function well. The characteristic luminosity at z = 0.5 is is found to be 16.30 mag, with an uncertainty of 10 per cent.