SILCC-Zoom: Polarisation and depolarisation in molecular clouds

We present synthetic dust polarisation 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 \(\mu\)m 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 depolarisation in the center of molecular clouds: We find that dust grains remain well aligned even at high densities (\(n\) \(>\) 10\(^3\) cm\(^{-3}\)) and visual extinctions (\(A_\text{V}\) \(>\) 1). The depolarisation 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\(^\circ\) 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 \(\lambda\) \(\gtrsim\) 160 \(\mu\)m the observed magnetic field traces reliably the underlying field in regions with intensities \(I\) \(\gtrsim\) 2 times the noise level and column densities above 1 M\(_\text{sun}\) pc\(^{-2}\). Here, typical deviations are \(\lesssim\) 10\(^\circ\). The observed structure is less reliable in regions with low polarisation 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 polarisation structure with deviations below 5\(^\circ\).