2012ApJ...744...85M

Using a mass-selected (M_{sstarf} ≥ 10¹¹ M_☉) sample of 198 galaxies at 0 ≤ z ≤ 3.0 with Hubble Space Telescope/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±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 modeling. The redshift evolution of the pair fraction is described by a power law F(z) = (0.07±0.04)x(1 + z)^0.6±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±0.5 major mergers 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.