Observational evidence that massive cluster galaxies were forming stars at z~2.5 and did not grow in mass at later times

Using Spitzer 3.6 micron data we derived the luminosity function and the mass function of galaxies in five z>1.4 clusters selected to have a firm intracluster medium detection. The five clusters differ in richness (ISCSJ1438.1+3414 and XMMXCSJ2215.9-1738 are twice as rich as ISCSJ1432.4+3250, IDCSJ1426.5+3508, and JKCS041) and morphological appareance. At the median redshift z=1.5, from the 150 member galaxies of the five clusters, we derived a characteristic magnitude of 16.92+/-0.13 in the [3.6] band and a characteristic mass of lgM*=11.30+/-0.05 Msol. We find that the characteristic luminosity and mass does not evolve between z=1 and 1.4<z<1.8, directly ruling out ongoing mass assembly between these epochs because massive galaxies are already present up to z=1.8. Lower-redshift build-up epochs have already been ruled out by previous works, leaving only z>1.8 as a possible epoch for the mass build up. However, the observed values of m^* at very high redshift are too bright for galaxies without any star formation immediately preceding the observed redshift and therefore imply a star formation episode not earlier than z_f=2.5. For the first time, mass/luminosity functions are able to robustly distinguish tiny differences between formation redshifts and to set {\it upper} limits to the epoch of the last star-formation episode.