A diversity of starburst-triggering mechanisms in interacting galaxies and their signatures in CO emission

The physical origin of enhanced star formation activity in interacting galaxies remains an open question. Knowing whether starbursts are triggered by an increase of the quantity of dense gas or an increase of the star formation efficiency would improve our understanding of galaxy evolution and allow to transpose the results obtained in the local Universe to high redshift galaxies. In this paper, we analyze a parsec-resolution simulation of an Antennae-like model of interacting galaxies. We find that the interplay of physical processes has complex and important variations in time and space, through different combinations of mechanisms like tides, shear and turbulence. These can have similar imprints on observables like depletion time and CO emission. The densest gas only constitutes the tail of the density distribution of some clouds, but exists in large excess in others. The super-linearity of the star formation rate dependence on gas density implies that this excess translates into a reduction of depletion times, and thus leads to a deviation from the classical star formation regime, visible up to galactic scales. These clouds are found in all parts of the galaxies, but their number density varies from one region to the next, due to different cloud assembly mechanisms. Therefore, the dependence of cloud and star formation-related quantities (like CO flux and depletion time) on the scale at which they are measured also varies across the galaxies. We find that the $α_{\rm CO}$ conversion factor between the CO luminosity and molecular gas mass has even stronger spatial than temporal variations in a system like the Antennae. These results raise a number of cautionary notes for the interpretation of observations of unresolved star-forming regions, but also predict that the diversity of environments for star formation will be better captured by the future generations of instruments.