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We use passive gas tracer particles in an Arepo simulation of a dwarf spiralgalaxy to relate the Lagrangian evolution of star-forming gas parcels and theirH2 molecules to the evolution of their host giant molecular clouds. We findthat the median chemical lifetime of H2 is just 4 Myr, independent of thelifetime of its host molecular cloud, which may vary from 1 to 90 Myr, with asubstantial portion of all star formation in the galaxy occurring in relativelylong-lived clouds. The rapid ejection of gas from around young massive stars byearly stellar feedback is responsible for this short H2 survival time, drivingdown the density of the surrounding gas, so that its H2 molecules aredissociated by the interstellar radiation field. This ejection of gas from theH2-dominated state is balanced by the constant accretion of new gas from thegalactic environment, constituting a "competition model" for molecular cloudevolution. Gas ejection occurs at a rate that is proportional to the molecularcloud mass, so that the cloud lifetime is determined by the accretion rate,which may be as high as 4 x 10^4 Msol/Myr in the longest-lived clouds. Ourfindings therefore resolve the conflict between observations of rapid gasejection around young massive stars and observations of long-lived molecularclouds in galaxies, that often survive up to several tens of Myr. We show thatthe fastest-accreting, longest-lived, highest-mass clouds drive supernovaclustering on sub-cloud scales, which in turn is a key driver of galacticoutflows.

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