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.
