The CFHTLS Deep Catalog of Interacting Galaxies I. Merger Rate Evolution
  to z=1.2
  We present the rest-frame optical galaxy merger fraction between 0.2<z<1.2,
as a function of stellar mass and optical luminosity, as observed by the
Canada-France-Hawaii Telescope Legacy Deep Survey (CFHTLS-Deep). We developed a
new classification scheme to identify major galaxy-galaxy mergers based on the
presence of tidal tails and bridges. These morphological features are signposts
of recent and ongoing merger activity. Through the visual classification of all
galaxies, down to i_vega<22.2 (~27,000 galaxies) over 2 square degrees, we have
compiled the CFHTLS Deep Catalog of Interacting Galaxies, with ~1600 merging
galaxies. We find the merger fraction to be 4.3% +/-0.3% at z~0.3 and 19.0%
+/-2.5% at z~1, implying evolution of the merger fraction going as (1+z)^m,
with m=2.25 +/-0.24. This result is inconsistent with a mild or non-evolving
(m<1.5) scenario at a >4sigma level of confidence. A mild trend, where massive
galaxies with M>10^10.7 M_sun are undergoing fewer mergers than less massive
systems M~10^10 M_sun), consistent with the expectations of galaxy assembly
downsizing is observed. Our results also show that interacting galaxies have on
average SFRs double that found in non-interacting field galaxies. We conclude
that (1) the optical galaxy merger fraction does evolve with redshift, (2) the
merger fraction depends mildly on stellar mass, with lower mass galaxies having
higher merger fractions at z<1, and (3) star formation is triggered at all
phases of a merger, with larger enhancements at later stages, consistent with
N-body simulations.
