Academia Sinica Logo

Abstract: Since the evolution of molecular clouds regulates star formation, which in turn drives galaxy evolution, the topic has been actively discussed since the 1970s, when direct observations of molecular clouds first became possible. In recent years, thanks to ALMA, a large number of nearby galaxies have been observed with spatial resolutions sufficient to resolve giant molecular clouds (GMCs) down to scales below 100 pc. These observations have provided empirical evolutionary frameworks and timescales for GMCs across galactic disks (e.g., Chevance et al. 2020; Demachi et al. 2024). However, even with such progress, the internal structures of GMCs have not yet been fully resolved, and the key physical processes that drive their evolution remain open questions. To address this, we have returned our focus to the Milky Way, aiming to establish a unified evolutionary framework by combining (1) wide-field, low-resolution surveys that trace the empirical evolution of molecular clouds, and (2) high-resolution observations of individual clouds that reveal the underlying physical processes. For (1), we focused on the outer solar circle, where line-of-sight contamination is relatively low. Using CO data obtained with the CfA 1.2-m and NANTEN2 4-m telescopes, we smoothed the maps to match the spatial resolution of extragalactic ALMA observations (~40 pc) and identified molecular clouds. The results show that the evolution of molecular clouds can be described by a scenario similar to that in external galaxies: over timescales of about 10 Myr, clouds grow in mass from ~10⁴ M☉ to ~10⁵ M☉ while becoming increasingly active in star formation. Furthermore, comparison with H I data suggests that Taurus-like dark clouds may evolve into Orion-like GMCs through HI gas accretion (Yamada, Ph.D. Thesis 2025). These findings imply that the transition from atomic to molecular gas is a fundamental process governing cloud evolution. Regarding the recent observational findings, which proposed GMC formation via inflowing H I gas onto the Galactic disk, we focus on the Draco cloud for (2), where interactions between infalling gas and the Galactic disk within the last few Myr have been suggested. Finally, to bridge (1) and (2), we will discuss a comprehensive analysis strategy using the FUGIN archival data, as well as future observations of deuterated species with the Nobeyama 45-m telescope and the 7-BEam Element (7-BEEs) receiver.