The XXL survey XV: Evidence for dry merger driven BCG growth in XXL-100-GC X-ray clusters

TL;DR

This study provides evidence that dry mergers are the primary mechanism driving the growth of brightest cluster galaxies (BCGs) in XXL-100-GC X-ray clusters at redshifts below 1, especially in relaxed systems.

Contribution

It demonstrates that BCG growth is mainly due to dry mergers rather than star formation, with the BCG mass maintaining a consistent fraction of the cluster mass in relaxed clusters.

Findings

BCGs in the sample have star formation levels similar to field ellipticals.

BCGs grow as their host clusters become more relaxed.

Post-merger, BCGs lag in stellar mass but catch up through dry mergers.

Abstract

The growth of brightest cluster galaxies is closely related to the properties of their host cluster. We present evidence for dry mergers as the dominant source of BCG mass growth at $z\lesssim1$ in the XXL 100 brightest cluster sample. We use the global red sequence, H$\alpha$ emission and mean star formation history to show that BCGs in the sample possess star formation levels comparable to field ellipticals of similar stellar mass and redshift. XXL 100 brightest clusters are less massive on average than those in other X-ray selected samples such as LoCuSS or HIFLUGCS. Few clusters in the sample display high central gas concentration, rendering inefficient the growth of BCGs via star formation resulting from the accretion of cool gas. Using measures of the relaxation state of their host clusters, we show that BCGs grow as relaxation proceeds. We find that the BCG stellar mass corresponds to a relatively constant fraction 1\%\ of the total cluster mass in relaxed systems. We also show that, following a cluster scale merger event, the BCG stellar mass lags behind the expected value from the M$_{cluster}$ - M$_{BCG}$ relation but subsequently accretes stellar mass via dry mergers as the BCG and cluster evolve towards a relaxed state.