Major dry mergers in early-type brightest cluster galaxies

Abstract

We search for ongoing major dry-mergers in a well selected sample of local Brightest Cluster Galaxies (BCGs) from the C4 cluster catalogue. 18 out of 515 early-type BCGs with redshift between 0.03 and 0.12 are found to be in major dry-mergers, which are selected as pairs (or triples) with $r$-band magnitude difference $\dm<1.5$ and projected separation $\rp<30$ kpc, and showing signatures of interaction in the form of significant asymmetry in residual images. We find that the fraction of BCGs in major dry-mergers increases with the richness of the clusters, consistent with the fact that richer clusters usually have more massive (or luminous) BCGs. We estimate that present-day early-type BCGs may have experienced on average $\sim 0.6 (\tmerge/0.3\Gyr)^{-1}$ major dry-mergers and through this process increases their luminosity (mass) by $15% (\tmerge/0.3\Gyr)^{-1} (\fmass/0.5)$ on average since $z=0.7$, where $\tmerge$ is the merging timescale and $\fmass$ is the mean mass fraction of companion galaxies added to the central ones. We also find that major dry-mergers do not seem to elevate radio activities in BCGs. Our study shows that major dry-mergers involving BCGs in clusters of galaxies are not rare in the local Universe, and they are an important channel for the formation and evolution of BCGs.