A Systematic Study of the Escape of LyC and Lyα Photons from Star-forming, Magnetized Turbulent Clouds
Taysun Kimm, Rebekka Bieri, Sam Geen, Joakim Rosdahl, J. Blaizot, L. Michel-Dansac, Thibault Garel
Abstract
Abstract Understanding the escape of Lyman continuum (LyC) and Ly α 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-magnetohydrodynamic 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 ⊙ ), as the clouds are dispersed within about 2–5 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 α shows a trend similar to that of LyC photons, except that the fraction of Ly α photons escaping from the GMCs is larger ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Ly</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>900</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>0.27</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) and that a GMC with strong turbulence shows larger f Ly α . The simulated GMCs show a characteristic velocity separation of Δ v ≈ 120 km s −1 in the time-averaged emergent Ly α spectra, suggesting that Ly α could be useful to infer the kinematics of the interstellar and circumgalactic medium. We show that Ly α 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 α , ultraviolet photons at 1500 Å, and the Balmer α line are weak.