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

Abstract

Brightest Cluster Galaxies (BCGs) might have been assembled relatively late\n(z<1) via mergers. By exploiting the high-resolution HST/ACS imaging, we find\nfour BCGs (COSMOS-P 125516, 102810, 036694 and 089357) in major dry merging in\n29 X-ray clusters at $0.3 \\le z \\le 0.6$ in the Cosmological Evolutionary\nSurvey (COSMOS). These BCGs show prominent but quiescent double nuclei with a\nmagnitude difference of $\\delta m<1.5$ and a projected separation of $r_p<$ 10\nkpc. Clear signatures of interaction such as extended plumes and/or significant\nasymmetries are also observed in their residual images. We infer a major merger\nrate of $0.55\\pm0.27$ merger per Gyr at $z\\sim0.43$ assuming the merger\ntime-scale estimate of Kitzbichler & White (2008). This inferred rate is\nsignificantly higher than the rate in the local Universe ($0.12\\pm0.03$ at\n$z\\sim0.07$) presented in Liu et al. (2009). We estimate that present-day BCGs\nincrease their luminosity (mass) by $\\sim35\\pm15$ per cent $(f_{mass}/0.5)$ via\nmajor dry mergers since $z=0.6$, where $f_{mass}$ is the mean mass fraction of\ncompanion galaxies accreted onto the central ones. Although the statistical\nuncertainty due to our small sample size is relatively large, our finding is\nconsistent with both recent observational and theoretical results. Furthermore,\nin conjunction with our previous findings in Liu et al. (2009), the discovery\nof these intermediate-redshift merging BCGs is clear evidence of ongoing\nassembly of BCGs via major dry mergers over the last $\\sim$6 Gyr.\n