Evolution of the galaxy merger rate in model universes

Abstract

We investigate the evolution of the galaxy merger rate predicted by two semi-analytical galaxy formation models implemented on the Millennium Simulation of dark matter structure growth. The fraction of merging galaxy pairs at each time-step of the simulation is derived from the galaxy catalogues obtained by the models and the results are compared with various observational estimates of merger fractions taken from the literature. We find a good match between the pair fractions derived from the simulation and the observed counting of galaxy pairs obtained by different sources in the redshift range 0 < z < 1.2. The predicted evolution of the number of galaxy mergers per Gyr grows with redshift as an exponential rate proportional to (1+z)^m, with m ranging from 0.6 to 0.8 for 0 < z < 2, depending on the luminosity and mass ratios of the merging galaxies. Our results are in agreement with recent observational results that argue for a flat evolution in the fraction of galaxy mergers since z ~ 1.2. The weak evolution predicted for the galaxy merger rate in an hierarchical model universe shows that the mass assembly evolution of galaxies through mergers does not follow the rapid evolution of the halo merger rate obtained in previous studies.