Self-consistent dust and non-LTE line radiative transfer with SKIRT

TL;DR

This paper presents an extension of the SKIRT 3D radiative transfer code to include non-LTE line calculations, enabling self-consistent modeling of molecular and atomic spectra alongside dust continuum in astrophysical environments.

Contribution

We developed and validated a new module for SKIRT that performs non-LTE line radiative transfer, allowing for comprehensive 3D modeling of dust and line emission in astrophysical objects.

Findings

Excellent agreement with benchmark results validates the implementation.

High-J CO lines are slightly attenuated by dust in edge-on views.

The method helps interpret observations from Herschel, ALMA, and JWST.

Abstract

We introduce Monte Carlo-based non-LTE line radiative transfer calculations in the 3D dust radiative transfer code SKIRT, which was originally set up as a dust radiative transfer code. By doing so, we develop a generic and powerful 3D radiative transfer code that can self-consistently generate spectra with molecular and atomic lines against the underlying continuum. We test the accuracy of the non-LTE line radiative transfer module in the extended SKIRT code using standard benchmarks. We find excellent agreement between the SKIRT results, the published benchmark results, and results obtained using the ray-tracing non-LTE line radiative transfer code MAGRITTE, which validates our implementation. We apply the extended SKIRT code on a 3D hydrodynamic simulation of a dusty AGN torus model and generate multi-wavelength images with CO rotational-line spectra against the underlying dust continuum. We find that the low-J CO emission traces the geometrically thick molecular torus, whereas the higher-J CO lines originate from the gas with high kinetic temperature located in the innermost regions of the torus. Comparing the calculations with and without dust radiative transfer, we find that higher-J CO lines are slightly attenuated by the surrounding cold dust when seen edge-on. This shows that atomic and molecular lines can experience attenuation, an effect that is particularly important for transitions at mid- and near-infrared wavelengths. Therefore, our self-consistent dust and non-LTE line radiative transfer calculations can help interpret the observational data from Herschel, ALMA, and JWST.