Close Pairs of Field Galaxies in the CNOC1 Redshift Survey

Abstract

A redshift sample of 545 field galaxies is used to examine how the galaxy merger rate changes with redshift and how mergers affect the observed properties of galaxies in close pairs. Close pairs are defined as those with projected separations less than 20 h-1 kpc. At a mean redshift of 0.33, it is found that 7.1% ? 1.4% of galaxies are in close physical pairs, compared to 4.3% ? 0.4% locally. The merger rate is estimated to change with redshift as (1 + z)2.8?0.9. These results are shown to be consistent with previous close-pair studies and provide one of the strongest constraints to date on the redshift dependence of the merger rate. As with earlier studies, no significant differences between the mean properties of paired and isolated galaxies are detected. However, using the subset of confirmed close physical pairs, those which appear to be undergoing interactions or mergers (based on their images) are found to have galaxies with strong emission lines and very blue rest-frame colors. In addition, these pairs have low relative velocities, increasing the likelihood that the galaxies will merge. We interpret this as the first clear evidence of merger-induced star formation occurring in field galaxies at this redshift.