The formation history of massive cluster galaxies as revealed by CARLA

TL;DR

This study investigates the formation and evolution of massive cluster galaxies from redshift 3.2 to 1.3 using CARLA survey data, revealing their star formation history, assembly timescales, and evolutionary sequence.

Contribution

It provides new insights into the formation timescales, star formation cessation, and assembly processes of massive cluster galaxies at high redshift.

Findings

Massive galaxies increase in abundance with decreasing redshift.

Star formation peaks at redshift 2-3 and terminates rapidly.

Massive galaxies at z>2 assembled within 0.5 Gyr, limiting dry merger contributions.

Abstract

We use a sample of 37 of the densest clusters and protoclusters across $1.3 \le z \le 3.2$ from the Clusters Around Radio-Loud AGN (CARLA) survey to study the formation of massive cluster galaxies. We use optical $i'$-band and infrared 3.6$\mu$m and 4.5$\mu$m images to statistically select sources within these protoclusters and measure their median observed colours; $\langle i'-[3.6] \rangle$. We find the abundance of massive galaxies within the protoclusters increases with decreasing redshift, suggesting these objects may form an evolutionary sequence, with the lower redshift clusters in the sample having similar properties to the descendants of the high redshift protoclusters. We find that the protocluster galaxies have an approximately unevolving observed-frame $i'-[3.6]$ colour across the examined redshift range. We compare the evolution of the $\langle i'-[3.6] \rangle$ colour of massive cluster galaxies with simplistic galaxy formation models. Taking the full cluster population into account, we show that the formation of stars within the majority of massive cluster galaxies occurs over at least 2Gyr, and peaks at $z \sim 2$-3. From the median $i'-[3.6]$ colours we cannot determine the star formation histories of individual galaxies, but their star formation must have been rapidly terminated to produce the observed red colours. Finally, we show that massive galaxies at $z>2$ must have assembled within 0.5Gyr of them forming a significant fraction of their stars. This means that few massive galaxies in $z>2$ protoclusters could have formed via dry mergers.