The Pair Fraction of Massive Galaxies at 0 < z < 3
  Using a mass-selected ($M_{\star} \ge 10^{11} M_{\odot}$) sample of 198
galaxies at 0 < z < 3.0 with HST/NICMOS $H_{160}$-band images from the COSMOS
survey, we find evidence for the evolution of the pair fraction above z ~ 2, an
epoch in which massive galaxies are believed to undergo significant structural
and mass evolution. We observe that the pair fraction of massive galaxies is
0.15 \pm 0.08 at 1.7 < z < 3.0, where galaxy pairs are defined as massive
galaxies having a companion of flux ratio from 1:1 to 1:4 within a projected
separation of 30 kpc. This is slightly lower, but still consistent with the
pair fraction measured previously in other studies, and the merger fraction
predicted in halo-occupation modelling. The redshift evolution of the pair
fraction is described by a power law F(z) = (0.07 \pm 0.04) * (1+z) ^ (0.6 \pm
0.5). The merger rate is consistent with no redshift evolution, however it is
difficult to constrain due to the limited sample size and the high
uncertainties in the merging timescale. Based on the merger rate calculation,
we estimate that a massive galaxy undergoes on average 1.1 \pm 0.5 major merger
from z = 3 to 0. The observed merger fraction is sufficient to explain the
number density evolution of massive galaxies, but insufficient to explain the
size evolution. This is a hint that mechanism(s) other than major merging may
be required to increase the sizes of the massive, compact quiescent galaxies
from z ~ 2 to 0.
