Clouds of Theseus: long-lived molecular clouds are composed of short-lived H2 molecules

We use passive gas tracer particles in an Arepo simulation of a dwarf spiral galaxy to relate the Lagrangian evolution of star-forming gas parcels and their H-2 molecules to the evolution of their host giant molecular clouds. We find that the median chemical lifetime of H-2 is 4 Myr, with an interquartile range between 2 and 9 Myr. This chemical lifetime is independent of the lifetime of the host molecular cloud, which may extend up to 90 Myr, with around 50 per cent of star formation occurring in longer lived clouds (>25 Myr). The rapid ejection of gas from around young massive stars by early stellar feedback is responsible for the short H-2 survival time, driving down the density of the surrounding gas, so that its H-2 molecules are dissociated by the interstellar radiation field. This ejection of gas from the H-2-dominated state is balanced by the constant accretion of new gas from the galactic environment, constituting a 'competition model' for molecular cloud evolution. Gas ejection occurs at a rate that is proportional to the molecular cloud mass, so that the cloud lifetime is determined by the accretion rate, which may be as high as 4 x 10(4) M-circle dot Myr(-1) in the longest lived clouds. Our findings therefore resolve the conflict between observations of rapid gas ejection around young massive stars and observations of long-lived molecular clouds in galaxies. We show that the fastest-accreting, longest lived, highest mass clouds drive supernova clustering on sub-cloud scales, which in turn is a key driver of galactic outflows.