Spatial kinematics of Brightest Cluster Galaxies and their close companions from Integral Field Unit spectroscopy

Abstract

We present Integral Field Unit (IFU) spectroscopy of four brightest cluster galaxies (BCGs) at z ~ 0.1. Three of the BCGs have close companions within a projected radius of 20 kpc and one has no companion within that radius.We calculate the dynamical masses of the BCGs and their companions to be 1.4 × 1011 < Mdyn(M⊙) < 1.5 × 1012. We estimate the probability that the companions of the BCGs are bound using the observed masses and velocity offsets. We show that the lowest mass companion (1:4) is not bound while the two nearly equal mass (1:1.45 and 1:1.25) companions are likely to merge with their host BCGs in 0.35 Gyr in major, dry mergers. We conclude that some BCGs continue to grow from major merging even at z ~ 0. We analyse the stellar kinematics of these systems using the λR parameter developed by the SAURON team. This offers a new and unique means to measure the stellar angular momentum of BCGs and make a direct comparison to other early-type galaxies. The BCGs and their companions have similar ellipticities to those of other early-type galaxies but are more massive. We find that not all these massive galaxies have low λRe as one might expect. One of the four BCGs and the two massive companions are found to be fast-rotating galaxies with high angular momentum, thereby providing a new test for models of galaxy evolution and the formation of intracluster light.