Interpreting Lyman $\alpha$ radiation from young, dusty galaxies

TL;DR

This paper introduces a numerical radiative transfer code for Lyα photons in galaxies, enabling detailed analysis of their escape, dust effects, and intergalactic medium interactions, to better interpret high-redshift galaxy observations.

Contribution

The paper presents a novel, flexible radiative transfer code for Lyα emission, accounting for complex galactic environments and dust, advancing the interpretation of high-redshift galaxy observations.

Findings

Calculated Lyα escape fractions in simulated galaxies.

Analyzed dust's impact on Lyα line profiles.

Explored Lyα transfer effects through the intergalactic medium.

Abstract

The significance of the Ly$\alpha$ emission line as a probe of the high-redshift Universe has long been established. Originating mainly in the vicinity of young, massive stars and in association with accretion of large bulks of matter, it is ideal for detecting young galaxies, the fundamental building blocks of our Universe. Since many different processes shape the spectrum and the spatial distribution of the Ly$\alpha$ photons in various ways, a multitude of physical properties of galaxies can be unveiled. However, this also makes the interpretation of Ly$\alpha$ observations notoriously difficult. Because Ly$\alpha$ is a resonant line, it scatters on neutral hydrogen, having its path length from the source to our telescopes vastly increased, and taking it through regions of unknown physical conditions. In this work, a numerical code capable of calculating realistically the radiative transfer of Ly$\alpha$ is presented. The code is capable of performing the radiative transfer in an arbitrary and adaptively refined distribution of Ly$\alpha$ source emission, temperature and velocity field of the interstellar and intergalactic medium, as well as density of neutral and ionized hydrogen, and, particularly important, dust. Accordingly, it is applied to galaxies simulated at high resolution, yielding a number of novel and interesting results, most notably the escape fractions of Ly$\alpha$ photons, the effect of dust on the line profile, and the impact of the transfer through the intergalactic medium. Furthermore, the remarkable detection of Ly$\alpha$ emission from a so-called "damped Ly$\alpha$ absorber" -- a special type of objects thought to be the progenitor of present-day's galaxies -- is presented, and the potential of the code for interpreting observations is demonstrated.