SILCC-Zoom: Polarization and depolarization in molecular clouds

Abstract

We present synthetic dust polarization maps of 3D magneto-hydrodynamical simulations of molecular clouds before the onset of stellar feedback. The clouds are modelled within the SILCC-Zoom project and are embedded in their galactic environment. The radiative transfer is carried out with POLARIS for wavelengths from 70 μm to 3 mm at a resolution of 0.12 pc, and includes self-consistently calculated alignment efficiencies for radiative torque alignment. We explore the reason of the observed depolarization in the center of molecular clouds: We find that dust grains remain well aligned even at high densities (n > 103 cm−3) and visual extinctions (AV > 1). The depolarization is rather caused by strong variations of the magnetic field direction along the LOS due to turbulent motions. The observed magnetic field structure thus resembles best the mass-weighted, line-of-sight averaged field structure. Furthermore, it differs by only a few 1◦ for different wavelengths and is little affected by the spatial resolution of the synthetic observations. Noise effects can be reduced by convolving the image. Doing so, for λ ≳ 160 μm the observed magnetic field traces reliably the underlying field in regions with intensities I ≳ 2 times the noise level and column densities above 1 M☉ pc−2. Here, typical deviations are ≲10◦. The observed structure is less reliable in regions with low polarization degrees and possibly in regions with large column density gradients. Finally, we show that a simplified and widely used method without self-consistent dust alignment efficiencies can provide a good representation of the observable polarization structure with deviations below 5◦