THE DRIVING MECHANISM OF STARBURSTS IN GALAXY MERGERS

Abstract

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