The merger rate of massive galaxies

We calculate the projected two-point correlation function for samples of luminous and massive galaxies in the COMBO-17 photometric redshift survey, focusing particularly on the amplitude of the correlation function at small projected radii and exploring the constraints such measurements can place on the galaxy merger rate. For nearly volume-limited samples with 0.4 < z < 0.8, we find that 4% ± 1% of luminous M_B < -20 galaxies are in close physical pairs (with real-space separation of <30 proper kpc). The corresponding fraction for massive galaxies with M* > 2.5×10 ¹⁰ M_⊙ is 5% ± 1%. Incorporating close pair fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid evolution of the merger fraction of massive galaxies between z = 0.8 and the present day. Assuming that the major merger timescale is of order the dynamical timescale for close massive galaxy pairs, we tentatively infer that ∼50% (70%) of all galaxies with present-day masses M* > 5 × 10 ¹⁰ M⊙ (remnants of mergers between galaxies with M* > 2.5 × 10¹⁰ M_⊙) have undergone a major merger since z = 0.8(1): major mergers between massive galaxies are a significant driver of galaxy evolution over the last eight billion years.