Ongoing growth of the brightest cluster galaxies via major dry mergers in the last ~6 Gyr

TL;DR

This study presents evidence that Brightest Cluster Galaxies (BCGs) have been actively assembling through major dry mergers over the last approximately 6 billion years, with a higher merger rate at intermediate redshifts than today.

Contribution

First direct evidence of ongoing major dry mergers in BCGs at z~0.4, quantifying the merger rate and its evolution over the last 6 Gyr.

Findings

Merger rate at z~0.43 is 0.55±0.27 per Gyr, higher than local Universe.

BCGs have increased their luminosity and mass by about 35% since z=0.6.

Ongoing assembly of BCGs via major dry mergers confirmed over last ~6 Gyr.

Abstract

Brightest Cluster Galaxies (BCGs) might have been assembled relatively late (z<1) via mergers. By exploiting the high-resolution HST/ACS imaging, we find four BCGs (COSMOS-P 125516, 102810, 036694 and 089357) in major dry merging in 29 X-ray clusters at $0.3 \le z \le 0.6$ in the Cosmological Evolutionary Survey (COSMOS). These BCGs show prominent but quiescent double nuclei with a magnitude difference of $\delta m<1.5$ and a projected separation of $r_p<$ 10 kpc. Clear signatures of interaction such as extended plumes and/or significant asymmetries are also observed in their residual images. We infer a major merger rate of $0.55\pm0.27$ merger per Gyr at $z\sim0.43$ assuming the merger time-scale estimate of Kitzbichler & White (2008). This inferred rate is significantly higher than the rate in the local Universe ($0.12\pm0.03$ at $z\sim0.07$) presented in Liu et al. (2009). We estimate that present-day BCGs increase their luminosity (mass) by $\sim35\pm15$ per cent $(f_{mass}/0.5)$ via major dry mergers since $z=0.6$, where $f_{mass}$ is the mean mass fraction of companion galaxies accreted onto the central ones. Although the statistical uncertainty due to our small sample size is relatively large, our finding is consistent with both recent observational and theoretical results. Furthermore, in conjunction with our previous findings in Liu et al. (2009), the discovery of these intermediate-redshift merging BCGs is clear evidence of ongoing assembly of BCGs via major dry mergers over the last $\sim$6 Gyr.