The diminishing importance of major galaxy mergers at higher redshifts
  Using mass-selected galaxy samples from deep multiwavelength data we
investigate the incidence of close galaxy pairs between z=0.4-2. Many such
close pairs will eventually merge, and the pair fraction is therefore related
to the merger rate. Over this redshift range the mean pair fraction is
essentially constant (evolving as f_pair (1+z)^{-0.4 +/- 0.6}) with about
6+/-1% of massive galaxies having a 1:4 or greater companion within 30h^-1 kpc.
Assuming the timescale over which pairs merge is not a strong function of
redshift, this implies a similarly constant merger rate (per unit time) out to
z=2. Since about three times as much cosmic time passes at z<1 as between
z=1-2, this implies that correspondingly more mergers occur in the low-redshift
universe. When minor companions (1:10 mass ratio or greater) are included, the
pair fraction increases to ~20% and still does not evolve strongly with
redshift. We also use a rest-frame color criterion to select pairs containing
only quiescent galaxies (major "dry merger" progenitors), and find them to be
similarly rare and constant with 4-7% of massive quiescent galaxies exhibiting
a nearby companion. Thus, even though other studies find major mergers to be
relatively uncommon since z=1, our results suggest that few additional mergers
occur in the 1<z<2 range and other mechanisms may be required to explain the
mass and size growth of galaxies over this epoch.
