A Hubble Space Telescope snapshot survey of dynamically close galaxy pairs in the CNOC2 redshift survey

We compare the structural properties of two classes of galaxies at intermediate redshift: those in dynamically close galaxy pairs, and those that are isolated. Both samples are selected from the Canadian Network for Observational Cosmology field galaxy redshift survey (CNOC2) and have redshifts in the range 0.1< z< 0.6. Hubble Space Telescope WFPC2 images were acquired as part of a snapshot survey and were used to measure bulge fraction and asymmetry for these galaxies. We find that paired and isolated galaxies have identical distributions of bulge fractions. Conversely, we find that paired galaxies are much more likely to be asymmetric (R-T + R-A >= 0.13) than isolated galaxies. Assuming that half of these pairs are unlikely to be close enough to merge, we estimate that 40% +/- 11% of merging galaxies are asymmetric, compared with 9% +/- 3% of isolated galaxies. The difference is even more striking for strongly asymmetric (R-T + R-A >= 0.16) galaxies: 25% +/- 8% for merging galaxies versus 1% +/- 1% for isolated galaxies. We find that strongly asymmetric paired galaxies are very blue, with rest-frame B - R colors close to 0.80, compared with a mean (B - R)(0) of 1.24 for all paired galaxies. In addition, asymmetric galaxies in pairs have strong [O II] lambda 3727 emission lines. We conclude that close to half of the galaxy pairs in our sample are in the process of merging and that most of these mergers are accompanied by triggered star formation.