Comparing Polarised Synchrotron and Thermal Dust Emission in the Galactic Plane

TL;DR

This study compares polarised synchrotron and thermal dust emissions in the Galactic plane to refine models of the Galactic magnetic field, revealing differences in field ordering and suggesting complex structures in the inner Galaxy.

Contribution

It introduces a new comparison between dust and synchrotron polarisation data to better constrain the Galactic magnetic field structure, especially in the outer Galaxy.

Findings

Dust polarisation indicates more ordered fields in the outer Galaxy.

The synchrotron and dust data are incompatible with previous models.

Inner Galaxy requires a more complex magnetic field scenario.

Abstract

As the next step toward an improved large scale Galactic magnetic field model, we present a simple comparison of polarised synchrotron and thermal dust emission on the Galactic plane. We find that the field configuration in our previous model that reproduces the polarised synchrotron is not compatible with the WMAP 94 GHz polarised emission data. In particular, the high degree of dust polarisation in the outer Galaxy (90deg < l < 270deg) implies that the fields in the dust-emitting regions are more ordered than the average of synchrotron-emitting regions. This new dust information allows us to constrain the spatial mixing of the coherent and random magnetic field components in the outer Galaxy. The inner Galaxy differs in polarisation degree and apparently requires a more complicated scenario than our current model. In the scenario that each interstellar component (including fields and now dust) follows a spiral arm modulation, as observed in external galaxies, the changing degree of ordering of the fields in dust-emitting regions may imply that the dust arms and the field component arms are shifted as a varying function of Galacto-centric radius. We discuss the implications for how the spiral arm compression affects the various components of the magnetised interstellar medium but conclude that improved data such as that expected from the Planck satellite will be required for a thorough analysis.