Effects of Radiative Transfer on the Observed Anisotropy in MHD Turbulent Molecular Simulations

TL;DR

This study investigates how radiative transfer effects influence observed anisotropy in MHD turbulent molecular simulations, highlighting differences between optically thin and thick lines and the impact of Mach number.

Contribution

It extends previous work by including radiative transfer in optically thick regimes and analyzing its effects on anisotropy in molecular line observations.

Findings

Anisotropy in velocity centroids remains similar in optically thick and thin lines.

Integrated intensity maps become more anisotropic in optically thick lines due to smaller scale probing.

Higher sonic Mach numbers tend to reduce the observed anisotropy.

Abstract

We study the anisotropy of centroid and integrated intensity maps with synthetic observations. We perform post-process radiative transfer including the optically thick regime that was not covered in Hern\'andez-Padilla et al. (2020). We consider the emission in various CO molecular lines, that range from optically thin to optically thick ($\mathrm{^{12}CO}$, $\mathrm{^{13}CO}$, $\mathrm{C^{18}O}$, and $\mathrm{C^{17}O}$). The results for the velocity centroids are similar to those in the optically thin case. For instance, the anisotropy observed can be attributed to the Alfv\'en mode, which dominates over the slow and fast modes when the line of sight is at a high inclination with respect to the mean magnetic field. A few differences arise in the models with higher opacity, where some dependence on the sonic Mach number becomes evident. In contrast to the optically thin case, maps of integrated intensity become more anisotropic in optically thick lines. In this situation the scales probed are restricted, due to absorption, to smaller scales which are known to be more anisotropic. We discuss how the sonic Mach number can affect the latter results, with highly supersonic cases exhibiting a lower degree of anisotropy.