The extreme synchronicity of stellar ages of red galaxies in the JKCS041 cluster at z=2.2

TL;DR

This study reveals that red galaxies in the JKCS041 cluster at z=2.2 have a remarkably synchronized stellar age distribution, challenging existing galaxy formation models that predict more stochastic star formation histories.

Contribution

It provides the first measurement of a very narrow stellar age spread in high-redshift cluster galaxies, indicating early and uniform star formation.

Findings

Stellar age spread of 160+/-30 Myr in red galaxies at z=2.2

Red galaxies show highly synchronized star formation histories

Challenges current models of stochastic galaxy formation

Abstract

Above redshift z ~ 1.4, we known little or nothing about the stellar ages of red galaxies in clusters, yet at these high redshifts important changes are predicted by current renditions of galaxy formation models embedded in the standard hierarchical paradigm of structure formation. Red-sequence galaxies in the cluster JKCS041 at z=2.2 show a tight distribution in colour that indicates a star formation history that is highly synchronized across galaxies. Specifically, we measure a spread in stellar age of 160+/-30 Myr, in marked disagreement with the current understanding of how massive red galaxies form in clusters, i.e. if they are produced somewhat stochastically in merging episodes that sometimes involve gas, hence star formation. The existence of a tight distribution in colour when the universe was at one quarter of its current age implies that mechanisms that have not yet been implemented in current galaxy formation scenarios long ago began to shape the star formation history of red cluster galaxies.