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

¹ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
² Centro de Estudios de Física del Cosmos de Aragón, Plaza San Juan 1, planta 2, 44001 Teruel, Spain
e-mail: clsj@cefca.es
³ Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
⁴ IRAP, Université de Toulouse, UPS-OMP, Toulouse, France
⁵ IASF – INAF, via Bassini 15, 20133 Milano, Italy
⁶ ESO, Karl-Schwarzschild-Str.2, 85748 Garching b. München, Germany
⁷ INAF – IASFBO, via P. Gobetti 101, 40129 Bologna, Italy

Received: 24 August 2012
Accepted: 10 March 2013

Abstract

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 μ = L₂/L₁ ≥ 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^+15.2_-6.8%, 20.1^+8.0_-5.1%, and 22.0^+13.7_-7.3% 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^+0.084_-0.038 Gyr^-1, 0.147^+0.058_-0.037 Gyr^-1, and 0.127^+0.079_-0.042 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  ~35% of the star-forming galaxies with stellar masses = 10¹⁰ - 10^10.5 M_⊙ have undergone a major merger since z ~ 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 ~ 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 ~ 1.5, particularly on the build-up of the red sequence.