Tracing the lifecycle of gas clouds: PAH emission and molecular cloud evolution in nearby galaxies

Recent JWST mid-infrared (mid-IR) images, capturing emission from polycyclic aromatic hydrocarbons (PAHs) and dust, reveal detailed gas distributions in nearby galaxies shaped by large-scale dynamics and stellar feedback. By leveraging PHANGS-JWST Cycle 1 and PHANGS-MUSE observations, we characterize the lifecycle of gas clouds from the PAH-emitting phase to HII regions across 17 nearby galaxies. We achieve this by analyzing the spatial distributions of mid-IR emission (7.7-11.3um) relative to Halpha emission as a function of spatial scales. In most galaxies, we find that the lifetimes of gas clouds measured by mid-IR closely match those traced by CO. This suggests that once gas clouds form, they quickly provide enough shielding for stable CO formation, particularly in molecular gas-rich galaxies with near-solar metallicity. Radiation fields from HII regions illuminate the surrounding PAHs and dust, resulting in a significant overlap (~70%) of the Halpha-emitting phase and mid-IR emission. Interestingly, despite the strong correlation between PAH and CO emission, we identify 1 kpc-long molecular gas ridges in several galaxies that are over 10 times brighter in CO than expected based on PAH emission. These ridges, located in dynamically complex regions such as spiral arms and bar ends, demonstrate that interstellar PAH processing (shattering or coagulation) and large-scale shocks significantly influence the molecular gas and dust properties in these environments.