Simulating Star Clusters Across Cosmic Time: II. Escape Fraction of Ionizing Photons from Molecular Clouds

Chong-Chong He; Massimo Ricotti; Sam Geen

doi:10.1093/mnras/staa165

paper

Simulating Star Clusters Across Cosmic Time: II. Escape Fraction of Ionizing Photons from Molecular Clouds

arXiv:2001.06109 · doi:10.1093/mnras/staa165

Abstract

We calculate the hydrogen and helium-ionizing radiation escaping star-forming molecular clouds, as a function of the star cluster mass and compactness, using a set of high-resolution radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds. In these simulations, presented in He, Ricotti and Geen (2019), the formation of individual massive stars are well resolved, and their UV radiation feedback and lifetime on the main sequence are modeled self-consistently. We find that the escape fraction of ionizing radiation from molecular clouds, $⟨ f_{esc}^{MC} ⟩$ , decreases with increasing mass of the star cluster and with decreasing compactness. Molecular clouds with densities typically found in the local Universe have negligible $⟨ f_{esc}^{MC} ⟩$ , ranging between $0.5%$ to $5%$ . Ten times denser molecular clouds have $⟨ f_{esc}^{MC} ⟩ \approx 10% - 20%$ , while $100 \times$ denser clouds, which produce globular cluster progenitors, have $⟨ f_{esc}^{MC} ⟩ \approx 20% - 60%$ . We find that $⟨ f_{esc}^{MC} ⟩$ increases with decreasing gas metallicity, even when ignoring dust extinction, due to stronger radiation feedback. However, the total number of escaping ionizing photons decreases with decreasing metallicity because the star formation efficiency is reduced. We conclude that the sources of reionization at $z > 6$ must have been very compact star clusters forming in molecular clouds about $100 \times$ denser than in today's Universe, which leads to a significant production of old globular clusters progenitors.

17 pages, 13 figures, accepted for publication in MNRAS

Simulating Star Clusters Across Cosmic Time: II. Escape Fraction of Ionizing Photons from Molecular Clouds

Abstract

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