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Published February 1, 2010 | Published
Journal Article Open

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 ≳4σ level of confidence. A mild trend, where by massive galaxies with M_*>10^(10.7) M_☉ are undergoing fewer mergers than less massive systems (M_* ~ 10^(10) M_☉), 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.

Additional Information

© 2010 American Astronomical Society. Print publication: Issue 2 (2010 February 1); received 2009 March 31, accepted for publication 2009 December 9; published 2010 January 11. We thank Kevin Bundy, C. Borys, and Lee Armus for their contributions to this work and the anonymous referee for valuable comments that improved the clarity of the paper. Financial support for this work was provided in part by the Natural Sciences and Engineering Research Council (NSERC) and an Ontario Graduate Scholarship in Science and Technology. M.S. acknowledges support from the Royal Society. This paper is based on observations obtained with MegaPrime/ MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT). The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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