Formation of globular clusters induced by external ultraviolet radiation
  II: Three-dimensional radiation hydrodynamics simulations
  We explore the possibility of the formation of globular clusters under
ultraviolet (UV) background radiation. One-dimensional spherical symmetric
radiation hydrodynamics (RHD) simulations by Hasegawa et al. have demonstrated
that the collapse of low-mass (10^6-10^7 solar masses) gas clouds exposed to
intense UV radiation can lead to the formation of compact star clusters like
globular clusters (GCs) if gas clouds contract with supersonic infall
velocities. However, three-dimensional effects, such as the anisotropy of
background radiation and the inhomogeneity in gas clouds, have not been studied
so far. In this paper, we perform three-dimensional RHD simulations in a
semi-cosmological context, and reconsider the formation of compact star
clusters in strong UV radiation fields. As a result, we find that although
anisotropic radiation fields bring an elongated shadow of neutral gas, almost
spherical compact star clusters can be procreated from a "supersonic infall"
cloud, since photo-dissociating radiation suppresses the formation of hydrogen
molecules in the shadowed regions and the regions are compressed by UV heated
ambient gas. The properties of resultant star clusters match those of GCs. On
the other hand, in weak UV radiation fields, dark matter-dominated star
clusters with low stellar density form due to the self-shielding effect as well
as the positive feedback by ionizing photons. Thus, we conclude that the
"supersonic infall" under a strong UV background is a potential mechanism to
form GCs.
