  Starburst (SB) galaxies are a rare population of galaxies with star formation
rates (SFRs) greatly exceeding those of the majority of star-forming galaxies
with similar stellar mass. It is unclear whether these bursts are the result of
either especially large gas reservoirs or enhanced efficiencies in converting
gas into stars. Tidal torques resulting from gas-rich galaxy mergers are known
to enhance the SFR by funneling gas towards the centre. However, recent
theoretical works show that mergers do not always trigger a SB and not all SB
galaxies are interacting systems, raising the question of what drives a SB. We
analyse a large sample of SB galaxies and a mass- and redshift-matched sample
of control galaxies, drawn from the FIREbox cosmological volume at z=0-1. We
find that SB galaxies have both larger molecular gas fractions and shorter
molecular depletion times than control galaxies, but similar total gas masses.
Control galaxies evolve towards the SB regime by gas compaction in their
central regions, over timescales of about 70 Myr, accompanied by an increase in
the fraction of ultra-dense and molecular gas. The driving mechanism behind the
SB varies depending on the mass of the galaxy. Massive (Mstar > 1e10 Msun)
galaxies undergoing intense, long-lasting SBs are mostly driven by galaxy
interactions. Conversely, SBs in non-interacting galaxies are often triggered
by a global gravitational instability, that can result in a breathing mode in
low-mass galaxies.
