THE GREATER IMPACT OF MERGERS ON THE GROWTH OF MASSIVE GALAXIES: IMPLICATIONS FOR MASS ASSEMBLY AND EVOLUTION SINCE z ≃ 1

Using deep infrared observations conducted with the MOIRCS imager on the Subaru Telescope in the northern GOODS field combined with public surveys in GOODS-S, we investigate the dependence on stellar mass, M(*), and galaxy type of the close pair fraction (5 h(-1) kpc < r(sep) < 20 h(-1) kpc) and implied merger rate. In terms of combined depth and survey area, our publicly available mass-limited sample represents a significant improvement over earlier infrared surveys used for this purpose. In common with some recent studies, we find that the fraction of paired systems that could result in major mergers is low (similar to 4%) and does not increase significantly with redshift to z approximate to 1.2, with proportional to (1+z)(1.6 +/- 1.6). Our key finding is that massive galaxies with M(*) > 10(11) M(circle dot) are more likely to host merging companions than less massive systems (M(*) similar to 10(10)M(circle dot)). We find evidence for a higher pair fraction for red, spheroidal hosts compared to blue, late-type systems, in line with expectations based on clustering at small scales. The so-called dry mergers between early-type galaxies devoid of star formation ( SF) represent nearly 50% of close pairs with M(*) > 3 x 10(10) M(circle dot) at z similar to 0.5, but less than 30% at z similar to 1. This result can be explained by the increasing abundance of red, early-type galaxies at these masses. We compare the volumetric merger rate of galaxies with different masses to mass-dependent trends in galaxy evolution. Our results reaffirm the conclusion of Bundy et al. that major mergers do not fully account for the formation of spheroidal galaxies since z similar to 1. In terms of mass assembly, major mergers contribute little to galaxy growth below M(*) similar to 3 x 10(10) M(circle dot) but play a more significant role among galaxies with M(*) similar to greater than or similar to 10(11) M(circle dot) similar to 30% of which have undergone mostly dry mergers over the observed redshift range. Overall, the relatively rapid and recent coalescence of high-mass galaxies mirrors the expected hierarchical growth of halos and is consistent with recent model predictions, even if the top down suppression of SF and morphological evolution (i.e., downsizing) involves additional physical processes.