The large-scale ordered magnetic field in the Galactic halo and the Local Bubble

Abstract

All-sky synchrotron polarization observations are the primary probe of the Galactic large-scale ordered magnetic field, which includes both coherent and ordered random components. However, magnetic-field strength estimates from synchrotron observations depend on the assumed cosmic-ray electron model, which itself depends on the magnetic-field configuration and strength throughout the Galaxy. We aim to constrain the strength of the large-scale ordered magnetic field in the Galactic halo through a self-consistent treatment of cosmic-ray electrons with the latest coherent magnetic-field configuration inferred by Xu & Han from rotation measures corrected for local contamination. We compare the Planck 30 GHz synchrotron polarization maps with our large-scale synchrotron models that jointly treat cosmic rays and magnetic fields and are constrained by direct cosmic-ray measurements and multifrequency data. Since the inferred field strength depends on the halo size, we explore different cosmic-ray propagation halo sizes. We observe a significant large-scale synchrotron excess relative to model predictions, which requires an ordered toroidal magnetic-field component in the halo with a peak strength of 3.2-4.3 microG, depending on the assumed halo size. We also find that the cosmic-ray propagation model with a 4 kpc halo size is preferred by the Planck synchrotron polarization maps. This result shows that the ordered magnetic field is 4-6 times stronger than the coherent field inferred from rotation measures by Xu & Han, establishing the presence of an ordered random field in the Galactic halo that is not traced by rotation measures. Attributing the synchrotron excess entirely to the Local Bubble would require non-standard Local Bubble properties, disfavoring this interpretation.