The dust polarisation and magnetic field structure in the centre of NGC253 with ALMA

Abstract

Context. Magnetic fields (B fields) have an impact on galaxy evolution on 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.Aims. The primary aim of this study is to obtain a parsec-scale map of the 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 starclusters (SSCs), to understand their combined effects on the galaxy’s star formation and ISM.Methods. We used ALMA full polarisation data in Bands 4 (∼145 GHz) and 7 (∼345 GHz) with a resolution of ∼25 and ∼5 pc scale, respectively. The Stokes I, Q, and U maps of the two bands have been used to compute the polarised intensity (PI), polarisation fraction (PF), B field orientation on the plane of the sky, and dispersion angle function (𝒮) maps. We computed the pixel-by-pixel uncertainties of these maps taking into account the covariance between the Stokes parameters I, Q, and U. The uncertainty allowed us to detect values of PF as low as ∼0.1% with a S/N (signal-to-noise ratio) greater than 3. Through a spectral energy distribution (SED)-fitting analysis including archival data, we investigated the main emitting components that contribute to the total and polarised emission in several areas of the starburst region.Results. According to our SED-fitting analysis, the observed Band 4 emission is a combination of dust, synchrotron, and free-free components, while Band 7 traces only dust. The PF of the synchrotron component measures ∼2%, while that of the dust component is ∼0.3%. The B field orientation maps in both bands at a common resolution show that the same B field structure is traced by dust and synchrotron emission at scales ∼25 pc. The B field morphology suggests a coupling with the multiphase outflow, while the distribution of PF in Band 7 shows to be correlated with the presence of SSCs. We observed a significant anti-correlation between PF and column density in both Bands 4 and 7. A negative correlation between PF and 𝒮 was observed in Band 4 but was nearly absent in Band 7 at a 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.