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

Abstract: Polarization of interstellar dust emission is a powerful probe of dust properties and magnetic field structure. Yet studies of external galaxies are hampered by foreground dust contribution. The aim of this study is to separate the polarised signal from the Large Magellanic Cloud (LMC) from that of the Milky Way (MW) in order to construct a wide-field, spatially complete map of dust polarization using the Planck 353 GHz data. To estimate the foreground polarization direction, we used velocity gradients in HI spectral line data and assessed the performance of the output by comparing to starlight extinction polarization. We estimate the foreground intensity using dust properties derived previously from the Planck data and we assume the foreground polarization to be uniform and equal to the average of the MW around the galaxy. After foreground removal, the geometry of the plane-of-the-sky magnetic field in the LMC tends to follow the structure of the atomic gas in the LMC. This is notably the case along the molecular ridges extending south and south-east of the 30 Doradus star-forming complex, and along more diffuse southern arm extending towards the Small Magellanic Cloud. There is also an alignment between the magnetic field and the outer arm in the western part. The median polarization fraction in the LMC is slightly lower than that observed for the MW while the anti-correlation between the polarization angle dispersion function and the polarization fraction is slightly larger. Overall, polarization fraction distribution is similar to that observed in the MW.