Evolution of the galaxy merger rate in model universes

TL;DR

This study compares galaxy merger rates from semi-analytical models with observations, finding a weak evolution consistent with a flat merger fraction since redshift 1.2, challenging previous rapid evolution predictions.

Contribution

It demonstrates that galaxy merger rates evolve slowly over time, aligning models with observations and contrasting with earlier halo merger rate studies.

Findings

Good match between simulation pair fractions and observations (0 < z < 1.2).

Merger rate grows exponentially with redshift, rate proportional to (1+z)^m.

Weak evolution suggests galaxy mass assembly via mergers is slower than halo merger predictions.

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.