A consistent measure of the merger histories of massive galaxies using close-pair statistics - I. Major mergers at z < 3.5

We use a large sample of ∼350 000 galaxies constructed by combining the UKIDSS UDS, VIDEO/CFHT-LS, UltraVISTA/COSMOS and GAMA survey regions to probe the major (1:4 stellar mass ratio) merging histories of massive galaxies (>10¹⁰ M_⊙) at 0.005 < z < 3.5. We use a method adapted from that presented in López-Sanjuan et al., using the full photometric redshift probability distributions, to measure pair fractions of flux-limited, stellar mass selected galaxy samples using close-pair statistics. The pair fraction is found to weakly evolve as ∝ (1 + z)^0.8 with no dependence on stellar mass. We subsequently derive major merger rates for galaxies at >10¹⁰ M_⊙ and at a constant number density of n > 10^-4 Mpc^-3, and find rates a factor of 2-3 smaller than previous works, although this depends strongly on the assumed merger time-scale and likelihood of a close-pair merging. Galaxies undergo approximately 0.5 major mergers at z < 3.5, accruing an additional (1-4) × 10¹⁰ M_⊙ in the process. On average, this represents an increase in stellar mass of 20-30 per cent (40-70 per cent) for constant stellar mass (constant number density) samples. Major merger accretion rate densities of ∼2 × 10^-4 M_⊙ yr^-1 Mpc^-3 are found for number density selected samples, indicating that direct progenitors of local massive (>10¹¹ M_⊙) galaxies have experienced a steady supply of stellar mass via major mergers throughout their evolution. While pair fractions are found to agree with those predicted by the Henriques et al. semi-analytic model, the Illustris hydrodynamical simulation fails to quantitatively reproduce derived merger rates. Furthermore, we find that major mergers become a comparable source of stellar mass growth compared to star formation at z < 1, but is 10-100 times smaller than the star formation rate density at higher redshifts.