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$^\circ$.