Context. Processes driving mass assembly are expected to evolve on different timescales along cosmic time. A transition might happen around z ∼ 1 as the cosmic star formation rate starts its decrease. Aims. We aim to identify the dynamical nature of galaxies in a representative sample to be able to infer and compare the mass assembly mechanisms across cosmic time.Methods. We present an analysis of the kinematics properties of 50 galaxies with redshifts 0.9 < z < 1.6 from the MASSIV sample observed with SINFONI/VLT with a mass range from 4.5 × 10 9 M to 1.7 × 10 11 M and a star formation rate from 6 M yr −1 to 300 M yr −1 . This is the largest sample with 2D kinematics in this redshift range. We provide a classification based on kinematics as well as on close galaxy environment. Results. We find that a significant fraction of galaxies in our sample (29%) experience merging or have close companions that may be gravitationally linked. This places a lower limit on the fraction of interacting galaxies because ongoing mergers are probably also present but harder to identify. We find that at least 44% of the galaxies in our sample display ordered rotation, whereas at least 35% are non-rotating objects. All rotators except one are compatible with rotation-dominated (V max /σ > 1) systems. Non-rotating objects are mainly small objects (R e < 4 kpc). They show an anti-correlation of their velocity dispersion and their effective radius. These lowmass objects (log M star < 10.5) may be ongoing mergers in a transient state, galaxies with only one unresolved star-forming region, galaxies with an unstable gaseous phase or, less probably, spheroids. Combining our sample with other 3D-spectroscopy samples, we find that the local velocity dispersion of the ionized gas component decreases continuously from z ∼ 3 to z = 0. The proportion of disks also seems to be increasing in star-forming galaxies when the redshift decreases. The number of interacting galaxies seems to be at a maximum at z ∼ 1.2. Conclusions. These results draw a picture in which cold gas accretion may still be efficient at z ∼ 1.2 but in which mergers may play a much more significant role at z ∼ 1.2 than at higher redshift. From a dynamical point of view, the redshift range 1 < z < 2 therefore appears as a transition period in the galaxy mass assembly process .
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 = 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 +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 M = 10 10 −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 gasrich mergers make an important contribution to the growth of massive galaxies since z ∼ 1.5, particularly on the build-up of the red sequence.
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