2012A&A...539A..92E

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. 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. 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 4.5x10⁹M_☉ to 1.7x10¹¹M_☉ and a star formation rate from 6M_☉/yr to 300M_☉/yr. 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. 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<4kpc). They show an anti-correlation of their velocity dispersion and their effective radius. These low-mass objects (logM_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. 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.