It's not easy being green: The evolution of galaxy colour in the EAGLE simulation

TL;DR

This study uses the EAGLE simulation to analyze how galaxy colours evolve over time, revealing the processes behind the formation of the red sequence and the typical timescales of colour transition.

Contribution

It provides new insights into galaxy colour evolution, quenching mechanisms, and characteristic evolutionary tracks in a cosmological simulation.

Findings

Median u-r colour of star-forming galaxies reddens by 1 mag from z=2 to 0.

Red sequence begins to form around z=1 due to quenching processes.

Most galaxies spend less than 2 Gyr in the green valley.

Abstract

We examine the evolution of intrinsic u-r colours of galaxies in the EAGLE cosmological hydrodynamical simulations, which has been shown to reproduce the observed redshift z=0.1 colour-magnitude distribution well. The median u-r of star-forming ('blue cloud') galaxies reddens by 1 mag from z=2 to 0 at fixed stellar mass, as their specific star formation rates decrease with time. A red sequence starts to build-up around z=1, due to the quenching of low-mass satellite galaxies at the faint end, and due to the quenching of more massive central galaxies by their active galactic nuclei (AGN) at the bright end. This leaves a dearth of intermediate-mass red sequence galaxies at z=1, which is mostly filled in by z=0. We quantify the time-scales of colour transition due to satellite and AGN quenching, finding that most galaxies spend less than 2 Gyr in the 'green valley'. On examining the trajectories of galaxies in a colour-stellar mass diagram, we identify three characteristic tracks that galaxies follow (quiescently star-forming, quenching and rejuvenating galaxies) and quantify the fraction of galaxies that follow each track.