  Galaxy interactions and mergers play a significant, but still debated and
poorly understood role in the star formation history of galaxies. Numerical and
theoretical models cannot yet explain the main properties of merger-induced
starbursts, including their intensity and their spatial extent. Usually, the
mechanism invoked in merger-induced starbursts is a global inflow of gas
towards the central kpc, resulting in a nuclear starburst. We show here, using
high-resolution AMR simulations and comparing to observations of the gas
component in mergers, that the triggering of starbursts also results from
increased ISM turbulence and velocity dispersions in interacting systems. This
forms cold gas that are denser and more massive than in quiescent disk
galaxies. The fraction of dense cold gas largely increases, modifying the
global density distribution of these systems, and efficient star formation
results. Because the starbursting activity is not just from a global compacting
of the gas to higher average surface densities, but also from higher turbulence
and fragmentation into massive and dense clouds, merging systems can enter a
different regime of star formation compared to quiescent disk galaxies. This is
in quantitative agreement with recent observations suggesting that disk
galaxies and starbursting systems are not the low-activity end and
high-activity end of a single regime, but actually follow different scaling
relations for their star formation.
