The Bright End of the Colour-Magnitude Relation

TL;DR

This study combines simulations and models to explore the origin of the colour-magnitude relation in early-type cluster galaxies, revealing that minor dry mergers increase galaxy mass without affecting their colours.

Contribution

It provides a detailed analysis of the processes influencing the colour-magnitude relation, especially highlighting the role of minor dry mergers in galaxy evolution.

Findings

Simulated and observed CMR show good general agreement.

Luminous galaxies deviate from linear CMR, showing nearly constant colours.

Minor dry mergers significantly increase galaxy mass without changing colours.

Abstract

We investigate the origin of the colour-magnitude relation (CMR) followed by early-type cluster galaxies by using a combination of cosmological N-body simulations of cluster of galaxies and a semi-analytic model of galaxy formation (Lagos, Cora & Padilla 2008). Results show good agreement between the general trend of the simulated and observed CMR. However, in many clusters, the most luminous galaxies depart from the linear fit to observed data displaying almost constant colours. With the aim of understanding this behaviour, we analyze the dependence with redshift of the stellar mass contributed to each galaxy by different processes, i.e., quiescent star formation, and starburst during major/minor and wet/dry mergers, and disk instability events. The evolution of the metallicity of the stellar component, contributed by each of these processes, is also investigated. We find that the major contribution of stellar mass at low redshift is due to minor dry merger events, being the metallicity of the stellar mass accreted during this process quite low. Thus, minor dry merger events seem to increase the mass of the more luminous galaxies without changing their colours.