TY - JOUR

T1 - THE IMPACT of UNRESOLVED TURBULENCE on the ESCAPE FRACTION of LYMAN CONTINUUM PHOTONS

AU - Safarzadeh, M.

AU - Scannapieco, Evan

N1 - Funding Information:
This work was supported by the National Science Foundation under grant AST14-07835 and by NASA under theory grant NNX15AK82G.

PY - 2016/11/20

Y1 - 2016/11/20

N2 - We investigate the relation between the turbulent Mach number (M) and the escape fraction of Lyman continuum photons ( fesc) in high-redshift galaxies. Approximating the turbulence as isothermal and isotropic, we show that the increase in the variance in column densities from M = 1 to M = 10 causes fesc to increase by ≈25%, and the increase from M = 1 to M = 20 causes fesc to increases by ≈50% for a medium with opacity t = 1. At a fixed Mach number, the correction factor for escape fraction relative to a constant column density case scales exponentially with the opacity in the cell, which has a large impact for simulated star-forming regions. Furthermore, in simulations of isotropic turbulence with full atomic/ionic cooling and chemistry, the fraction of HI drops by a factor of ≈2.5 at M ≈ 10 even when the mean temperature is ≈ 5 × 103 K. If turbulence is unresolved, these effects together enhance fesc by a factor > 3 at Mach numbers above 10. Such Mach numbers are common at high redshifts where vigorous turbulence is driven by supernovae, gravitational instabilities, and merger activity, as shown both by numerical simulations and observations. These results, if implemented in the current hydrodynamical cosmological simulations to account for unresolved turbulence, can boost the theoretical predictions of the Lyman Continuum photon escape fraction and further constrain the sources of reionization.

AB - We investigate the relation between the turbulent Mach number (M) and the escape fraction of Lyman continuum photons ( fesc) in high-redshift galaxies. Approximating the turbulence as isothermal and isotropic, we show that the increase in the variance in column densities from M = 1 to M = 10 causes fesc to increase by ≈25%, and the increase from M = 1 to M = 20 causes fesc to increases by ≈50% for a medium with opacity t = 1. At a fixed Mach number, the correction factor for escape fraction relative to a constant column density case scales exponentially with the opacity in the cell, which has a large impact for simulated star-forming regions. Furthermore, in simulations of isotropic turbulence with full atomic/ionic cooling and chemistry, the fraction of HI drops by a factor of ≈2.5 at M ≈ 10 even when the mean temperature is ≈ 5 × 103 K. If turbulence is unresolved, these effects together enhance fesc by a factor > 3 at Mach numbers above 10. Such Mach numbers are common at high redshifts where vigorous turbulence is driven by supernovae, gravitational instabilities, and merger activity, as shown both by numerical simulations and observations. These results, if implemented in the current hydrodynamical cosmological simulations to account for unresolved turbulence, can boost the theoretical predictions of the Lyman Continuum photon escape fraction and further constrain the sources of reionization.

KW - galaxies: ISM

KW - galaxies: high-redshift

KW - radiative transfer

KW - turbulence

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U2 - 10.3847/2041-8205/832/1/L9

DO - 10.3847/2041-8205/832/1/L9

M3 - Article

AN - SCOPUS:84996671028

VL - 832

JO - Astrophysical Journal Letters

JF - Astrophysical Journal Letters

SN - 2041-8205

IS - 1

M1 - L9

ER -