Polarised emission from aligned dust grains in nearby galaxies: Predictions from the Auriga simulations

Abstract

Context. Polarised emission from non-spherical dust grains contains information about the alignment of these dust grains and traces the structure of the interstellar magnetic field. Methods. We post-processed 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 constrained our dust model using Planck 353 GHz observations of the Milky Way. This model was 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 ≈7% for face-on galaxies and ≈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.

Unveiling polarized emission from interstellar dust of the Large Magellanic Cloud with Planck

“…We obtained an average value of the LMC polarization fraction of 5.1%. We use results from recent simulations by Vandenbroucke et al (2021) to show that this is lower than the expected value for a galaxy similar to ours observed faced-on at the distance of the LMC. The maximum polarization value we observe is however significantly higher than the expected value from those simulations (13%).…”

“…XIX 2015), the difference in column density may explain the slightly lower maximum polarization fraction observed here for the LMC. Vandenbroucke et al (2021) simulated polarimetric observations of external galaxies, including dust properties, alignment, radiative transfer and spiral magnetic field geometries with different inclination angles, based on the MHD structure predicted by the Auriga simulations. The model, when observed from within the simulated galaxy, was constrained using the Planck data and was used to derive linear polarization fractions that could have been observed with the SPICA/B-BOP instrument.…”

“…Their derived value of the lower limit for the maximum polarization fraction at this resolution is around 13% and the average value of the polarisation fraction is around 6%. As stated by Vandenbroucke et al (2021), to determine the maximum polarization fraction of an external galaxy, it should be resolved with at least 1 kpc, and the LMC observations with Planck largely satisfy this requirement. The values we derive here for the polarization fraction of the LMC are therefore slightly lower than the expectations for the average value and significantly higher for the maximum value.…”

Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- IV: Program overview and first results on the polarization fraction

“…The increase in their polarized spectrum is expected as the optical depth decreases with wavelength, producing an increase in thermal polarized emission. Interestingly, Vandenbroucke et al (2021) showed a polarized spectrum with a dip in the 60−90 µm wavelength range. They assumed a linear mixture of perfectly aligned silicate dust grains and non-aligned graphite dust grains.…”

О Связи T-Матриц, Возникающих В Осесимметричной Задаче Рассеяния Света Сфероидом

“…Представление реальных рассеивателей сфероидами является подходом, широко применяемым в разных областях науки: например, в оптике атмосферы [1][2][3], астрономии [4], медицине [5], нанооптике [6], лабораторном анализе [7] и т. д. Особенно часто используется сфероидальная модель в современной астрофизике из-за отсутствия информации о форме несферических космических пылинок [8][9][10][11][12].…”