THE MOLECULAR CLOUD LIFECYCLE II: FORMATION AND DESTRUCTION OF MOLECULAR CLOUDS DIAGNOSED VIA H2 FLUORESCENT EMISSION

Molecular hydrogen (H2) formation and dissociation are key processes that drive the gas lifecycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds (SILCC) zoom-in simulation suite, we explore the utility of future observations of H2 dissociation and formation for tracking the lifecycle of molecular clouds. The simulations used in this work include non-equilibrium H2 formation, stellar radiation, sink particles, and turbulence. We find that, at early times in the cloud evolution, H2 formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H2 formation is also associated with a higher early star formation rate. For the clouds studied here, H2 is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady-state about 2 Myrs 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 for the molecular cloud lifecycle and the star formation efficiency may be distinguished with observational measurements of H2 fluorescence with a space- based high-resolution FUV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H2 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, H2 chemical equilibrium, and cloud destruction.