  Context. Identifying the main processes of galaxy assembly at high redshifts
is still a major issue to understand galaxy formation and evolution at early
epochs in the history of the Universe. Aims. This work aims to provide a first
insight into the dynamics and mass assembly of galaxies at redshifts 1.2<z<1.6,
the early epoch just before the sharp decrease of the cosmic star formation
rate. Methods. We use the near-infrared integral field spectrograph SINFONI on
the ESO-VLT under 0.65 seeing to obtain spatially resolved spectroscopy on nine
emission line galaxies with 1.2<z<1.6 from the VIMOS VLT Deep Survey. We derive
the velocity fields and velocity dispersions on kpc scales using the Halpha
emission line. Results. Out of the nine star-forming galaxies, we find that
galaxies distribute in three groups: two galaxies can be well reproduced by a
rotating disk, three systems can be classified as major mergers and four
galaxies show disturbed dynamics and high velocity dispersion. We argue that
there is evidence for hierarchical mass assembly from major merger, with most
massive galaxies with M>10^11Msun subject to at least one major merger over a 3
Gyr period as well as for continuous accretion feeding strong star formation.
Conclusions. These results point towards a galaxy formation and assembly
scenario which involves several processes, possibly acting in parallel, with
major mergers and continuous gas accretion playing a major role. Well
controlled samples representative of the bulk of the galaxy population at this
key cosmic time are necessary to make further progress.
