Ly-alpha Radiative Transfer in Cosmological Simulations and Application to a z~8 Emitter

Abstract

We develop a Ly-alpha radiative transfer (RT) Monte Carlo code for cosmological simulations.High resolution,along with appropriately treated cooling can result in simulated environments with very high optical depths.Thus,solving the Ly-alpha RT problem in cosmological simulations can take an unrealistically long time.For this reason,we develop methods to speed up the Ly-alpha RT.With these accelerating methods,along with the parallelization of the code,we make the problem of Ly-alpha RT in the complex environments of cosmological simulations tractable.We test the RT code against simple Ly-alpha emitter models,and then we apply it to the brightest Ly-alpha emitter of a gasdynamics+N-body Adaptive Refinement Tree (ART) simulation at z~8.We find that recombination rather than cooling radiation Ly-alpha photons is the dominant contribution to the intrinsic Ly-alpha luminosity of the emitter,which is ~4.8x10e43 ergs/s.The size of the emitter is pretty small,making it unresolved for currently available instruments.Its spectrum before adding the Ly-alpha Gunn-Peterson absorption (GP) resembles that of static media,despite some net inward radial peculiar motion.This is because for such high optical depths as those in ART simulations,velocities of order some hundreds km/s are not important.We add the GP in two ways.First we assume no damping wing,corresponding to the situation where the emitter lies within the HII region of a very bright quasar,and second we allow for the damping wing.Including the damping wing leads to a maximum line brightness suppression by roughly a factor of ~62.The line fluxes,even though quite faint for current ground-based telescopes,should be within reach for JWST.