The Impact of Stellar Radiative Feedback on Formation of Young Massive Clusters via Fast H I Gas Collisions

Young massive clusters (YMCs) are dense aggregates of young stars and are often speculated as potential precursors to globular clusters. However, the formation mechanism of massive and compact gas clumps that precede YMCs remains unknown. In this paper, we study the formation of such massive clumps via fast H I gas collisions (∼100 km s^‑1) as suggested by recent observations and their subsequent evolution into YMCs by using three-dimensional magnetohydrodynamics simulations involving self-gravity and detailed thermal/chemical processes. In particular, the impact of ionization feedback from stellar radiation is included in an approximate fashion where the temperature within the H II regions is elevated to 10,000 K, while supernova feedback is not included. We examine whether the resulting massive clumps can survive this ionization feedback and evolve into YMCs. Our simulations reveal the emergence of gas clumps that not only possess substantial mass (∼10⁵ M _⊙) but also sufficient compactness (∼5 pc). Notably, these clumps exhibit significantly higher escape velocities compared to the sound speed of the H II region, indicating effective gravitational retention of gas against feedback-induced evaporation. Consequently, these conditions foster efficient star formation within the massive gas clumps, ultimately leading to their evolution into YMCs. We also perform simulations involving lower-velocity gas collisions, approximately 15 km s^‑1, typical shock velocities induced by galactic superbubbles. In contrast to the high-velocity collisions, we find that molecular cloud formation does not occur in the case of 1 cm^‑3 gas collision, while YMC formation is observed in the presence of denser gas of 10 cm^‑3. However, the formation of YMCs requires compression periods exceeding 10 Myr in these cases, indicating a potential preference for gas collisions driven by intergalactic interactions rather than galactic superbubbles for YMC formation.