ART2: coupling Lyα line and multi-wavelength continuum radiative transfer

Abstract

Narrow-band Lyα line and broad-band continuum have played important roles in the discovery of high-redshift galaxies in recent years. Hence, it is crucial to study the radiative transfer of both Lyα and continuum photons in the context of galaxy formation and evolution in order to understand the nature of distant galaxies. Here, we present a three-dimensional Monte Carlo radiative transfer code, All-wavelength Radiative Transfer with Adaptive Refinement Tree (ART2), which couples Lyα line and multi-wavelength continuum, for the study of panchromatic properties of galaxies and interstellar medium. This code is based on the original version of Li et al., and features three essential modules: continuum emission from X-ray to radio, Lyα emission from both recombination and collisional excitation, and ionization of neutral hydrogen. The coupling of these three modules, together with an adaptive refinement grid, enables a self-consistent and accurate calculation of the Lyα properties, which depend strongly on the UV continuum, ionization structure and dust content of the object. Moreover, it efficiently produces multi-wavelength properties, such as the spectral energy distribution and images, for direct comparison with multi-band observations. As an example, we apply ART2 to a cosmological simulation that includes both star formation and black hole growth, and study in detail a sample of massive galaxies at redshifts z = 3.1–10.2. We find that these galaxies are Lyα emitters (LAEs), whose Lyα emission traces the dense gas region, and that their Lyα lines show a shape characteristic of gas inflow. Furthermore, the Lyα properties, including photon escape fraction, emergent luminosity and equivalent width, change with time and environment. Our results suggest that LAEs evolve with redshift, and that early LAEs such as the most distant one detected at z ∼ 8.6 may be dwarf galaxies with a high star formation rate fuelled by infall of cold gas, and a low Lyα escape fraction.