Star-forming disk galaxies at high redshift are often subject to violent disk instability, characterized by giant clumps whose fate is yet to be understood. The main question is whether the clumps disrupt within their dynamical timescale (≤ 50 Myr), like the molecular clouds in today's galaxies, or whether they survive stellar feedback for more than a disk orbital time (≈ 300 Myr) in which case they can migrate inward and help building the central bulge. We present 3.5-7 pc resolution AMR simulations of high-redshift disks including photo-ionization, radiation pressure, and supernovae feedback. Our modeling of radiation pressure determines the mass loading and initial velocity of winds from basic physical principles. We find that the giant clumps produce steady outflow rates comparable to and sometimes somewhat larger than their star formation rate, with velocities largely sufficient to escape galaxy. The clumps also lose mass, especially old stars, by tidal stripping, and the stellar populations contained in the clumps hence remain relatively young (≤ 200 Myr), as observed. The clumps survive gaseous outflows and stellar loss, because they are wandering in gas-rich turbulent disks from which they can re-accrete gas at high rates compensating for outflows and tidal stripping, overall keeping realistic and self-regulated gaseous and stellar masses. Our simulations produce gaseous outflows with velocities, densities and mass loading consistent with observations, and at the same time suggest that the giant clumps survive for hundreds of Myr and complete their migration to the center of highredshift galaxies, without rapid dispersion and reformation of clumps. These long-lived clumps can be involved in inside-out evolution and thickening of the disk, spheroid growth and fueling of the central black hole.

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 .

Galaxy mergers are believed to trigger strong starbursts. This is well assessed by observations in the local Universe. However the efficiency of this mechanism has poorly been tested so far for high redshift, actively star forming, galaxies. We present a suite of pc-resolution hydrodynamical numerical simulations to compare the star formation process along a merging sequence of high and low z galaxies, by varying the gas mass fraction between the two models. We show that, for the same orbit, high-redshift gasrich mergers are less efficient than low-redshift ones at producing starbursts: the star formation rate excess induced by the merger and its duration are both around 10 times lower than in the low gas fraction case. The mechanisms that account for the star formation triggering at low redshift -the increased compressive turbulence, gas fragmentation, and central gas inflows -are only mildly, if not at all, enhanced for high gas fraction galaxy encounters. Furthermore, we show that the strong stellar feedback from the initially high star formation rate in high redshift galaxies does not prevent an increase of the star formation during the merger. Our results are consistent with the observed increase of the number of major mergers with increasing redshift being faster than the respective increase in the number of starburst galaxies.