2021ApJ...908....2M

Young massive clusters (YMCs) are dense aggregates of young stars, which are essential to galaxy evolution, owing to their ultraviolet radiation, stellar winds, and supernovae. The typical mass and radius of YMCs are M ∼ 10⁴ M_☉ and R ∼ 1 pc, respectively, indicating that many stars are located in a small region. The formation of YMC precursor clouds may be difficult because a very compact massive cloud should be formed before stellar feedback blows off the cloud. Recent observational studies suggest that YMCs can be formed as a consequence of the fast H I gas collision with a velocity of ∼100 km s^–1, which is the typical velocity of the galaxy-galaxy interaction. In this study, we examine whether the fast H I gas collision triggers YMC formation using three-dimensional magnetohydrodynamics simulations, which includes the effects of self-gravity, radiative cooling/heating, and chemistry. We demonstrate that massive gravitationally bound gas clumps with M > 10⁴ M_☉ and L ∼ 4 pc are formed in the shock compressed region induced by the fast H I gas collision, in which massive gas clumps can evolve into YMCs. Our results show that the YMC precursors are formed by the global gravitational collapse of molecular clouds, and YMCs can be formed even in low-metal environments, such as the Magellanic Clouds. Additionally, the very massive YMC precursor cloud, with M > 10⁵ M_☉, can be created when we consider the fast collision of H I clouds, which may explain the origin of the very massive stellar cluster R136 system in the Large Magellanic Cloud.