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 (approximate to 27,000 galaxies) over 2 square degrees, we have compiled the CFHTLS-Deep Catalog of Interacting Galaxies, with approximate to 1600 merging galaxies. We find the merger fraction to be 4.3% +/- 0.3% at z similar to 0.3 and 19.0% +/- 2.5% at z similar to 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 greater than or similar to 4 sigma level of confidence. A mild trend, where by massive galaxies with M* > 10(10.7) M(circle dot) are undergoing fewer mergers than less massive systems (M* similar to 10(10) M(circle dot)), 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.