The Dust Polarisation and Magnetic Field Structure in the Centre of NGC253 with ALMA

Abstract: Magnetic fields have an impact on galaxy evolution at multiple scales. They are particularly important for starburst galaxies, where they play a crucial role in shaping the interstellar medium (ISM), influencing star formation processes and interacting with galactic outflows. The primary aim of this study is to obtain a parsec scale map of dust polarisation and B-field structure within the central starburst region of NGC253. This includes examining the relationship between the morphology of B-fields, galactic outflows and the spatial distribution of super star clusters (SSC), to understand their combined effects on the galaxy's star formation and ISM. We used ALMA full polarisation data in Bands 4 (145 GHz) and 7 (345 GHz) with resolution of 25 and 5 pc scale, respectively. According to our SED fitting analysis, the observed Band 4 emission is a combination of dust, synchrotron and free-free, while Band 7 traces only dust. The polarisation fraction (PF) of the synchrotron component is 2%, while that of the dust component is 0.3%. The B-fields orientation maps in both bands at common resolution show that the same B-fields structure is traced by dust and synchrotron emission at scales of 25 pc. The B-field morphology suggests a coupling with the multiphase outflow, while the distribution of PF in Band 7 showed to be correlated with the presence of SSC. We observed a significant anti-correlation between polarisation fraction and column density in both Bands 4 and 7. A negative correlation between PF and dispersion angle function was observed in Band 4 but was nearly absent in Band 7 at native resolution, suggesting that the tangling of B-field geometry along the plane of the sky is the main cause of depolarisation at 25 pc scales, while other factors play a role at 5 pc scales.