The Major and Minor Galaxy Merger Rates at z < 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-M20, 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 range of mass-ratio measured by
different techniques and differing parent galaxy selection. The major merger
rate per unit co-moving volume for samples selected with constant number
density evolves much more strongly with redshift (~ (1+z)^(+3.0 \pm 1.1)) than
samples selected with constant stellar mass or passively evolving luminosity (~
(1+z)^(+0.1 \pm 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-M20. The implied minor
merger rate is ~3 times the major merger rate at z ~ 0.7, and shows little
evolution with redshift.
