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$^\circ$.
