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We propose a simple analytic theory for environmentally-dependent molecularcloud lifetimes, based on the large-scale (galactic) dynamics of theinterstellar medium. Within this theory, the cloud lifetime is set by thetime-scales for gravitational collapse, galactic shear, spiral arminteractions, epicyclic perturbations and cloud-cloud collisions. It isdependent on five observable quantities, accessible through measurements of thegalactic rotation curve, the gas and stellar surface densities, and the gas andstellar velocity dispersions of the host galaxy. We determine how the relativeimportance of each dynamical mechanism varies throughout the space ofobservable galactic properties, and conclude that gravitational collapse andgalactic shear play the greatest role in setting the cloud lifetime for theconsidered range of galaxy properties, while cloud-cloud collisions exert amuch lesser influence. All five environmental mechanisms are neverthelessrequired to obtain a complete picture of cloud evolution. We apply our theoryto the galaxies M31, M51, M83, and the Milky Way, and find a strong dependenceof the cloud lifetime upon galactocentric radius in each case, with a typicalcloud lifetime between 10 and 50 Myr. Our theory is ideally-suited forsystematic observational tests with the Atacama Large Millimetre/submillimetrearray.

A general theory for the lifetimes of giant molecular clouds under the influence of galactic dynamics