The driving mechanism of starbursts in galaxy mergers
  We present hydrodynamic simulations of a major merger of disk galaxies, and
study the ISM dynamics and star formation properties. High spatial and mass
resolutions of 12pc and 4x10^4 M_sol allow to resolve cold and turbulent gas
clouds embedded in a warmer diffuse phase. We compare to lower resolution
models, where the multiphase ISM is not resolved and is modeled as a relatively
homogeneous and stable medium. While merger-driven bursts of star formation are
generally attributed to large-scale gas inflows towards the nuclear regions, we
show that once a realistic ISM is resolved, the dominant process is actually
gas fragmentation into massive and dense clouds and rapid star formation
therein. As a consequence, star formation is more efficient by a factor of up
to 10 and is also somewhat more extended, while the gas density probability
distribution function (PDF) rapidly evolves towards very high densities. We
thus propose that the actual mechanism of starburst triggering in galaxy
collisions can only be captured at high spatial resolution and when the cooling
of gas is modeled down to less than 10^3 K. Not only does our model reproduce
the properties of the Antennae system, but it also explains the ``starburst
mode'' revealed recently in high-redshift mergers compared to quiescent disks.
