Formation of Massive Star Clusters by Fast HI Gas Collision
  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~10^4 M_sun 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 HI 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 HI gas collision triggers YMC formation using
three-dimensional magnetohydrodynamics simulations, which include the effects
of self-gravity, radiative cooling/heating, and chemistry. We demonstrate that
massive gravitationally bound gas clumps with M >10^4 M_sun and L~4 pc are
formed in the shock compressed region induced by the fast HI gas collision,
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 >10^5 M_sun,
can be created when we consider the fast collision of HI clouds, which may
explain the origin of the very massive stellar cluster R136 system in the Large
Magellanic Cloud.
