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 $\alpha_{\rm CO}$ 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.
