Polarised emission from aligned dust grains in nearby galaxies:
  predictions from the Auriga simulations
  (Aims) In this work, we predict the far-infrared polarisation signal emitted
by non-spherical dust grains in nearby galaxies. We determine the angular
resolution and sensitivity required to study the magnetic field configuration
in these galaxies. (Methods) We post-process a set of Milky Way like galaxies
from the Auriga project, assuming a dust mix consisting of spheroidal dust
grains that are partially aligned with the model magnetic field. We constrain
our dust model using Planck 353 GHz observations of the Milky Way. This model
is then extrapolated to shorter wavelengths that cover the peak of interstellar
dust emission and to observations of arbitrarily oriented nearby Milky Way like
galaxies. (Results) Assuming an intrinsic linear polarisation fraction that
does not vary significantly with wavelength for wavelengths longer than 50
micron, we predict a linear polarisation fraction with a maximum of $10-15\%$
and a median value of $\approx{}7\%$ for face-on galaxies and $\approx{}3\%$
for edge-on galaxies. The polarisation fraction anti-correlates with the line
of sight density and with the angular dispersion function which expresses the
large scale order of the magnetic field perpendicular to the line of sight. The
maximum linear polarisation fraction agrees well with the intrinsic properties
of the dust model. The true magnetic field orientation can be traced along low
density lines of sight when it is coherent along the line of sight. These
results also hold for nearby galaxies, where a coherent magnetic field
structure is recovered over a range of different broad bands. (Conclusions)
Polarised emission from non-spherical dust grains accurately traces the large
scale structure of the galactic magnetic field in Milky Way like galaxies, with
expected maximum linear polarisation fractions of $10-15\%$. To resolve this
maximum, a spatial resolution of at least 1 kpc is required.
