The driving mechanism of starbursts in galaxy mergers

TL;DR

High-resolution hydrodynamic simulations reveal that starbursts in galaxy mergers are primarily triggered by gas fragmentation into dense clouds, rather than large-scale inflows, emphasizing the importance of detailed ISM modeling.

Contribution

This study demonstrates that resolving the multiphase ISM at high spatial resolution changes the understanding of starburst mechanisms in galaxy mergers.

Findings

Star formation is up to 10 times more efficient with high-resolution ISM modeling.

Gas density PDFs evolve rapidly towards very high densities during mergers.

The model reproduces properties of the Antennae system and explains high-redshift starburst modes.

Abstract

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.