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 on the Subaru
Telescope in GOODS-N combined with public surveys in GOODS-S, we investigate
the dependence on stellar mass, M_*, and galaxy type of the close pair fraction
(5 kpc < r < 20 kpc) and implied merger rate. In common with some recent
studies we find that the fraction of paired systems that could result in major
mergers is low (~4%) and does not increase significantly with redshift to
z~1.2, with (1+z)^{1.6 \pm 1.6}. Our key finding is that massive galaxies with
M_* > 1E11 Msun are more likely to host merging companions than less massive
systems (M_* ~ 1E10 Msun). 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. So-called "dry" mergers between early-type
galaxies represent nearly 50% of close pairs with M_* > 3E10 Msun at z~0.5, but
less than 30% at z~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, finding that major mergers cannot fully account for the formation of
spheroidal galaxies since z~1. In terms of mass assembly, major mergers
contribute little to galaxy growth below M_* ~ 3E10 Msun but are more
significant among galaxies with M_* > 1E11 Msun, 30% of which have undergone
mostly dry mergers over the observed redshift range. Overall, the relatively
more rapid coalescence of high mass galaxies mirrors the expected hierarchical
growth of halos and is consistent with recent model predictions, even if the
downsizing of star formation and morphological evolution involves additional
physical processes.
