Lyman-alpha transmission properties of the intergalactic medium in the CoDaII simulation

Abstract

The decline in abundance of Lyman-$\alpha$ (Ly$\alpha$) emitting galaxies at $z\gtrsim 6$ is a powerful and commonly used probe to constrain the progress of cosmic reionization. We use the CoDaII simulation, which is a radiation hydrodynamic simulation featuring a box of $\sim 94$ comoving Mpc side length, to compute the Ly$\alpha$ transmission properties of the intergalactic medium (IGM) at $z\sim 5.8$ to $7$. Our results mainly confirm previous studies, i.e., we find a declining Ly$\alpha$ transmission with redshift and a large sightline-to-sightline variation. However, motivated by the recent discovery of blue Ly$\alpha$ peaks at high redshift, we also analyze the IGM transmission on the blue side, which shows a rapid decline at $z\gtrsim 6$ of the blue transmission. This low transmission can be attributed not only to the presence of neutral regions but also to the residual neutral hydrogen within ionized regions, for which a density even as low as $n_{\mathrm{HI}}\sim 10^{-9}{\mathrm{cm}}^{-3}$ (sometimes combined with kinematic effects) leads to a significantly reduced visibility. Still, we find that $\sim 1\%$ of sightlines towards $M_{1600AB}\sim -21$ galaxies at $z\sim 7$ are transparent enough to allow a transmission of a blue Ly$\alpha$ peak. We discuss our results in the context of the interpretation of observations.

13 pages, 12 figures. Published version

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