The Millennium Galaxy Catalogue: Dynamically Close Galaxy Pairs and the Global Merger Rate

Abstract

We derive the number of dynamically close companions per galaxy ($N_c$) and their total luminosity ($L_c$) for galaxies in the Millennium Galaxy Catalogue: $N_c$ is similar to the fraction of galaxies in close pairs and is directly related to the galaxy merger rate. We find $N_c=0.0174\pm 0.0015$ and $L_c=(252\pm 30)\times 10^6$ $L_{\odot }$ for galaxies with $-22<M_B-5\log h<-19$ with $<z>=0.123$ and $N_c=0.0357\pm 0.0027$, $L_c=(294\pm 31)\times 10^6{L}_{\odot }$ for galaxies with $-21<M_B-5\log h<-18$, with $<z>=0.116$. The integrated merger rate to $z=1$ for both samples is about 20 %, but this depends sensitively on the fraction of kinematic pairs that are truly undergoing a merger (assumed here to be 50%), the evolution of the merger rate (here as $(1+z{)}^3$) and the adopted timescale for mergers (0.2 and 0.5 Gyr for each sample, respectively). Galaxies involved in mergers tend to be marginally bluer than non-interacting galaxies and show an excess of both early-type and very late-type objects and a deficiency of intermediate-type spirals. This suggests that interactions and mergers partly drive the star formation and morphological evolution of galaxies.

26 pages, 3 figures, AJ accepted for publication