MASSIV: Mass Assembly Survey with SINFONI in VVDS II. Kinematics and close environment classification

Context. Processes driving mass assembly are expected to evolve on different timescales along cosmic time. A transition might happen around z similar to 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 x 10(9) M-circle dot to 1.7 x 10(11) M-circle dot and a star formation rate from 6 M-circle dot yr(-1) to 300 M-circle dot 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/sigma > 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 low-mass 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 similar to 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 similar to 1.2. Conclusions. These results draw a picture in which cold gas accretion may still be efficient at z similar to 1.2 but in which mergers may play a much more significant role at z similar to 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(star star star star).