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

TL;DR

This study uses three-dimensional radiation hydrodynamics simulations to investigate how external ultraviolet radiation influences the formation of globular clusters, revealing that strong UV fields can induce their formation through supersonic gas infall.

Contribution

First 3D RHD simulations demonstrating globular cluster formation via supersonic infall under strong UV radiation, considering anisotropic radiation and gas inhomogeneity effects.

Findings

Strong UV radiation can produce nearly spherical globular clusters.

Weak UV fields lead to dark matter-dominated, low-density star clusters.

Supersonic infall is a viable mechanism for globular cluster formation.

Abstract

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.