A close-pair analysis of damp mergers at intermediate redshifts

We have studied the kinematics of ~2800 candidate close pair galaxies at 0.1<z<1.2 identified from the Canada-France-Hawaii Telescope Legacy Survey fields. Spectra of these systems were obtained using spectrometers on the 6.5m Magellan and 5m Hale telescopes. These data allow us to constrain the rate of dry mergers at intermediate redshifts and to test the `hot halo' model for quenching of star formation. Using virial radii estimated from the correlation between dynamical and stellar masses published by Leauthaud et al. (2011), we find that around 1/5 of our candidate pairs are likely to share a common dark matter halo (our metric for close physical association). These pairs are divided into red-red, blue-red and blue-blue systems using the rest-frame colors classification method introduced in Chou et al. (2011). Galaxies classified as red in our sample have very low star-formation rates, but they need not be totally quiescent, and hence we refer to them as `damp', rather than `dry', systems. After correcting for known selection effects, the fraction of blue-blue pairs is significantly greater than that of red-red and blue-red pairs. Red-red pairs are almost entirely absent from our sample, suggesting that damp mergers are rare at z~0.5. Our data supports models with a short merging timescale (<0.5 Gyr) in which star-formation is enhanced in the early phase of mergers, but quenched in the late phase. Hot halo models may explain this behaviour, but only if virial shocks that heat gas are inefficient until major mergers are nearly complete.