V. The major merger rate of star-forming galaxies at 0.9 < z < 1.8 from IFS-based close pairs
Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de
Marseille) UMR 7326,
2 Centro de Estudios de Física del Cosmos de Aragón, Plaza San Juan 1, planta 2, 44001 Teruel, Spain
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
Accepted: 10 March 2013
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 (rp) 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 rp ≤ 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 - 1010.5 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.
Key words: galaxies: evolution / galaxies: interactions / galaxies: formation
This work is based mainly on observations collected at the European Southern Observatory (ESO) Very Large Telescope (VLT), Paranal, Chile, as part of the Programs 179.A-0823, 177.A-0837, 78.A-0177, 75.A-0318, and 70.A-9007. This work also benefits from data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2013