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

TL;DR

This study provides observational evidence that massive cluster galaxies formed their stars around redshift 2.5 and did not significantly grow in mass afterward, challenging previous assumptions about galaxy evolution.

Contribution

It offers the first robust constraints on the epoch of last star formation in massive cluster galaxies using luminosity and mass functions.

Findings

Massive galaxies are already in place up to z=1.8.

Characteristic luminosity and mass do not evolve between z=1 and 1.4<z<1.8.

Star formation episodes occurred not earlier than z=2.5.

Abstract

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.