The CFHTLS Deep Catalog of Interacting Galaxies I. Merger Rate Evolution to z=1.2

TL;DR

This study measures the galaxy merger fraction evolution from redshift 0.2 to 1.2 using CFHTLS data, revealing a significant increase with redshift and linking mergers to enhanced star formation, with a new classification scheme for identifying mergers.

Contribution

Introduces a novel morphological classification method based on tidal features to identify galaxy mergers in a large survey, and quantifies merger fraction evolution up to z=1.2.

Findings

Merger fraction increases from 4.3% at z~0.3 to 19.0% at z~1.

Merger fraction evolves as (1+z)^2.25, indicating significant evolution.

Interacting galaxies have double the star formation rates of non-interacting galaxies.

Abstract

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.