Lyman alpha Radiative Transfer with Dust: Escape Fractions from Simulated High-Redshift Galaxies

TL;DR

This study combines high-resolution simulations and advanced radiative transfer modeling to quantify how dust affects Lyman alpha photon escape from high-redshift galaxies, revealing dependencies on galaxy mass and spectral effects.

Contribution

It introduces a detailed Monte Carlo radiative transfer model with dust in cosmological simulations to analyze Lyman alpha escape fractions and spectral modifications.

Findings

Escape fraction decreases with galaxy mass, from near 100% at 10^9 M_sun to below 10% at 10^12 M_sun.

Emergent spectrum is non-uniformly affected, with line center photons escaping more readily.

Dust causes a narrowing of the Lyman alpha line, especially near the line center.

Abstract

The Lyman alpha emission line is an essential diagnostic tool for probing galaxy formation and evolution. Not only is it commonly the strongest observable line from high-redshift galaxies but from its shape detailed information about its host galaxy can be revealed. However, due to the scattering nature of Lya photons increasing their path length in a non-trivial way, if dust is present in the galaxy the line may be severely suppressed and its shape altered. In order to interpret observations correctly, it is thus of crucial significance to know how much of the emitted light actually escapes the galaxy. In the present work, using a combination of high-resolution cosmological hydro-simulations and an adaptively refinable Monte Carlo Lya radiative transfer code including an advanced model of dust, the escape fractions f_esc of Lya radiation from high-redshift (z = 3.6) galaxies are calculated. In addition to the average escape fraction, the variation of f_esc in different directions and from different parts of the galaxies is investigated, as well as the effect on the emergent spectrum. Escape fractions from a sample of simulated galaxies of representative physical properties are found to decrease for increasing galaxy virial mass M_vir, from f_esc approaching unity for M_vir ~ 10^9 M_sun to f_esc less than 10% for M_vir ~ 10^12 M_sun. In spite of the dust being almost grey, it is found that the emergent spectrum is affected non-uniformly, with the escape fraction of photons close to the line center being much higher than of those in the wings, thus effectively narrowing the Lya line.