The Molecular Cloud Life Cycle. II. Formation and Destruction of Molecular Clouds Diagnosed via H₂ Fluorescent Emission

Abstract Molecular hydrogen (H 2 ) formation and dissociation are key processes that drive the gas life cycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds zoom-in simulation suite, we explore the utility of future observations of H 2 dissociation and formation for tracking the life cycle of molecular clouds. The simulations used in this work include nonequilibrium H 2 formation, stellar radiation, sink particles, and turbulence. We find that at early times in the cloud evolution H 2 formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H 2 formation is also associated with a higher early star formation rate. For the clouds studied here, H 2 is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady state about 2 Myr after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories of the molecular cloud life cycle and star formation efficiency may be distinguished with observational measurements of H 2 fluorescence with a space-based high-resolution far-UV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H 2 dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud’s star-forming life, including cloud building, rapidly star forming, H 2 chemical equilibrium, and cloud destruction.