Major Dry-Mergers In Early-Type Brightest Cluster Galaxies

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 δmr < 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 ∼ 0.6 (tmerge/0.3Gyr) −1 major dry-mergers and through this process increases their luminosity (mass) by 15% (tmerge/0.3Gyr) −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.