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We explore the possibility of the formation of globular clusters underultraviolet (UV) background radiation. One-dimensional spherical symmetricradiation hydrodynamics (RHD) simulations by Hasegawa et al. have demonstratedthat the collapse of low-mass (10^6-10^7 solar masses) gas clouds exposed tointense UV radiation can lead to the formation of compact star clusters likeglobular clusters (GCs) if gas clouds contract with supersonic infallvelocities. However, three-dimensional effects, such as the anisotropy ofbackground radiation and the inhomogeneity in gas clouds, have not been studiedso far. In this paper, we perform three-dimensional RHD simulations in asemi-cosmological context, and reconsider the formation of compact starclusters in strong UV radiation fields. As a result, we find that althoughanisotropic radiation fields bring an elongated shadow of neutral gas, almostspherical compact star clusters can be procreated from a "supersonic infall"cloud, since photo-dissociating radiation suppresses the formation of hydrogenmolecules in the shadowed regions and the regions are compressed by UV heatedambient gas. The properties of resultant star clusters match those of GCs. Onthe other hand, in weak UV radiation fields, dark matter-dominated starclusters with low stellar density form due to the self-shielding effect as wellas the positive feedback by ionizing photons. Thus, we conclude that the"supersonic infall" under a strong UV background is a potential mechanism toform GCs.

Formation of globular clusters induced by external ultraviolet radiation II: Three-dimensional radiation hydrodynamics simulations