Star formation in galaxy mergers: ISM turbulence, dense gas excess, and scaling relations for disks and starbusts

TL;DR

This paper demonstrates that galaxy mergers trigger starbursts not only through gas inflow but also via increased turbulence and dense gas formation, leading to different star formation regimes compared to quiescent disks.

Contribution

It reveals that ISM turbulence and dense gas formation are key factors in merger-induced starbursts, expanding understanding beyond the traditional inflow model.

Findings

Mergers increase ISM turbulence and dense gas fractions.

Starbursts result from higher turbulence and cloud fragmentation.

Different scaling relations exist for star formation in disks and starbursts.

Abstract

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.