The Impact of Unresolved Turbulence on the Escape Fraction of Lyman Continuum Photons

Abstract

We investigate the relation between the turbulent Mach number (\mach) and the escape fraction of Lyman continuum photons ($f_{\rm esc}$) in high-redshift galaxies. Approximating the turbulence as isothermal and isotropic, we show that the increase in the variance in column densities from $\mathcal{M}=1$ to $\mathcal{M}=10$ causes $f_{\rm esc}$ to increase by $\approx 25$\%, and the increase from $\mathcal{M}=1$ to $\mathcal{M}=20$ causes $f_{\rm esc}$ to increases by $\approx 50$\% for a medium with opacity $\tau\approx1$. 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 $\approx 2.5$ at $\mathcal{M}\approx10$ even when the mean temperature is $\approx5\times10^3 K$. If turbulence is unresolved, these effects together enhance $f_{\rm esc}$ 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.