Observational evidence that massive cluster galaxies were forming stars atz~ 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 (ISCS J1438.1+3414 and XMMXCS J2215.9-1738 are twice as rich as ISCS J1432.4+3250, IDCS J1426.5+3508, and JKCS 041) 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 lg M∗ = 11.30 ± 0.05 M⊙. 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 zf = 2.5. For the first time, mass/luminosity functions are able to robustly distinguish tiny differences between formation redshifts and to set upper limits to the epoch of the last star-formation episode.