THE MAJOR AND MINOR GALAXY MERGER RATES ATz< 1.5

Calculating the galaxy merger rate requires both a census of galaxies identified as merger candidates and a cosmologically averaged "observability" timescale 〈 T obs ( z )〉 for identifying galaxy mergers. While many have counted galaxy mergers using a variety of techniques, 〈 T obs ( z )〉 for these techniques have been poorly constrained. We address this problem by calibrating three merger rate estimators with a suite of hydrodynamic merger simulations and three galaxy formation models. We estimate 〈 T obs ( z )〉 for (1) close galaxy pairs with a range of projected separations, (2) the morphology indicator G − M 20 , and (3) the morphology indicator asymmetry A . Then, we apply these timescales to the observed merger fractions at z < 1.5 from the recent literature. When our physically motivated timescales are adopted, the observed galaxy merger rates become largely consistent. The remaining differences between the galaxy merger rates are explained by the differences in the ranges of the mass ratio measured by different techniques and differing parent galaxy selection. The major merger rate per unit comoving volume for samples selected with constant number density evolves much more strongly with redshift (∝(1 + z ) +3.0 ± 1.1 ) than samples selected with constant stellar mass or passively evolving luminosity (∝(1 + z ) +0.1 ± 0.4 ). We calculate the minor merger rate (1:4 < M sat / M primary ≲ 1:10) by subtracting the major merger rate from close pairs from the "total" merger rate determined by G − M 20 . The implied minor merger rate is ∼3 times the major merger rate at z ∼ 0.7 and shows little evolution with redshift.