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 \pm 0.13 in the [ 3.6 ] band and a characteristic mass of lgM ^ { * } = 11.30 \pm 0.05 M _ { \odot } .
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 upper limits to the epoch of the last star-formation episode .