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