A systematic study of the escape of LyC and Ly$\alpha$ photons from star-forming, magnetized turbulent clouds

TL;DR

This study uses high-resolution simulations to investigate how LyC and Ly$ extalpha$ photons escape from giant molecular clouds, revealing dependencies on cloud properties and implications for galaxy spectra and reionization.

Contribution

It provides the first detailed radiation-magneto-hydrodynamic simulation analysis of LyC and Ly$ extalpha$ photon escape from GMCs, including star formation and feedback effects.

Findings

15-70% of ionizing radiation escapes from GMCs within 2-5 Myr.

Escape fractions are higher in less massive, metal-poor, less turbulent, and less dense clouds.

Ly$ extalpha$ escape correlates with LyC escape and shows characteristic spectral features.

Abstract

Understanding the escape of Lyman continuum (LyC) and Lyman $\alpha$ (Ly$\alpha$) photons from giant molecular clouds (GMCs) is crucial if we are to study the reionization of the Universe and to interpret spectra of observed galaxies at high redshift. To this end, we perform high-resolution, radiation-magneto-hydrodynamic simulations of GMCs with self-consistent star formation and stellar feedback. We find that a significant fraction (15-70%) of ionizing radiation escapes from the simulated GMCs with different masses ($10^5$ and $10^6\,M_\odot$), as the clouds are dispersed within about $2$-$5\,{\rm Myr}$ from the onset of star formation. The fraction of LyC photons leaked is larger when the GMCs are less massive, metal-poor, less turbulent, and less dense. The most efficient leakage of LyC radiation occurs when the total star formation efficiency of a GMC is about 20%. The escape of Ly$\alpha$ shows a trend similar to that of LyC photons, except that the fraction of Ly$\alpha$ photons escaping from the GMCs is larger ($f_{\rm esc}^{\rm Ly\alpha}\approx f_{900}^{0.27}$) and that a GMC with strong turbulence shows larger $f_{\rm esc}^{\rm Ly\alpha}$. The simulated GMCs show a characteristic velocity separation of $\Delta v\approx 120 \,{\rm km\,s^{-1}}$ in the time-averaged emergent Ly$\alpha$ spectra, suggesting that Ly$\alpha$ could be useful to infer the kinematics of the interstellar and circumgalactic medium. We show that Ly$\alpha$ luminosities are a useful indicator of the LyC escape, provided the number of LyC photons can be deduced through stellar population modeling. Finally, we find that the correlations between the escape fractions of Ly$\alpha$, ultraviolet photons at 1500A, and the Balmer $\alpha$ line are weak.