MASSIV: Mass Assembly Survey with SINFONI in VVDS V. The major merger rate of star-forming galaxies at 0.9 < z < 1.8 from IFS-based close pairs

Context. The contribution of the merging process to the early phase of galaxy assembly at z > 1 and, in particular, to the build-up of the red sequence, still needs to be accurately assessed. Aims. We aim to measure the major merger rate of star-forming galaxies at 0.9 < z < 1.8, using close pairs identified from integral field spectroscopy (IFS). Methods. We use the velocity field maps obtained with SINFONI/VLT on the MASSIV sample, selected from the star-forming population in the VVDS. We identify physical pairs of galaxies from the measurement of the relative velocity and the projected separation (r(p)) of the galaxies in the pair. Using the well constrained selection function of the MASSIV sample, we derive at a mean redshift up to z = 1.54 the gas-rich major merger fraction (luminosity ratio mu = L-2/L-1 1/4 >=), and the gas-rich major merger rate using merger time scales from cosmological simulations. Results. We find a high gas-rich major merger fraction of 20.8(-6.8)(+15.2)%, 20.1(-5.1)(+8.0)%, and 22.0(-7.3)(+13.7)% for close pairs with r(p) <= 20 h(-1) kpc in redshift ranges z = [0.94, 1.06], [1.2, 1.5), and [1.5, 1.8), respectively. This translates into a gas-rich major merger rate of 0.116(-0.038)(+0.084) Gyr(-1), 0.147(-0.037)(+0.058) Gyr(-1), and 0.127(-0.042)(+0.079) Gyr(-1) at z = 1.03, 1.32, and 1.54, respectively. Combining our results with previous studies at z < 1, the gas-rich major merger rate evolves as (1 + z)(n), with n = 3.95 +/- 0.12, up to z = 1.5. From these results we infer that similar to 35% of the star-forming galaxies with stellar masses <(M)over bar>(star) = 10(10) - 10(10.5) M-circle dot have undergone a major merger since z similar to 1.5. We develop a simple model that shows that, assuming that all gas-rich major mergers lead to early-type galaxies, the combined effect of gas-rich and dry mergers is able to explain most of the evolution in the number density of massive early-type galaxies since z similar to 1.5, with our measured gas-rich merger rate accounting for about two-thirds of this evolution. Conclusions. Merging of star-forming galaxies is frequent at around the peak in star formation activity. Our results show that gas-rich mergers make an important contribution to the growth of massive galaxies since z similar to 1.5, particularly on the build-up of the red sequence.