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

Abstract

We derive the number of dynamically close companions per galaxy (Nc) and their total luminosity (Lc) for galaxies in the Millennium Galaxy Catalogue. Nc is similar to the fraction of galaxies in close pairs and is directly related to the galaxy merger rate. We find Nc = 0.0174 ± 0.0015 and Lc = (252 ± 30) × 106 L⊙ for galaxies with -22 < MB - 5 log h < -19 and (z) = 0.123, and Nc = 0.0357 ± 0.0027 and Lc = (294 ± 31) × 106 L⊙ for galaxies with -21 < MB -5 log h < -18 and (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 noninteracting 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.