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Brightest Cluster Galaxies (BCGs) might have been assembled relatively late(z<1) via mergers. By exploiting the high-resolution HST/ACS imaging, we findfour BCGs (COSMOS-P 125516, 102810, 036694 and 089357) in major dry merging in29 X-ray clusters at $0.3 \le z \le 0.6$ in the Cosmological EvolutionarySurvey (COSMOS). These BCGs show prominent but quiescent double nuclei with amagnitude difference of $\delta m<1.5$ and a projected separation of $r_p<$ 10kpc. Clear signatures of interaction such as extended plumes and/or significantasymmetries are also observed in their residual images. We infer a major mergerrate of $0.55\pm0.27$ merger per Gyr at $z\sim0.43$ assuming the mergertime-scale estimate of Kitzbichler & White (2008). This inferred rate issignificantly 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 BCGsincrease their luminosity (mass) by $\sim35\pm15$ per cent $(f_{mass}/0.5)$ viamajor dry mergers since $z=0.6$, where $f_{mass}$ is the mean mass fraction ofcompanion galaxies accreted onto the central ones. Although the statisticaluncertainty due to our small sample size is relatively large, our finding isconsistent with both recent observational and theoretical results. Furthermore,in conjunction with our previous findings in Liu et al. (2009), the discoveryof these intermediate-redshift merging BCGs is clear evidence of ongoingassembly of BCGs via major dry mergers over the last $\sim$6 Gyr.

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