Comparing polarized synchrotron and thermal dust emission in the Galactic plane

As the next step towards an improved large-scale Galactic magnetic field model, we present a simple comparison of polarized synchrotron and thermal dust emission on the Galactic plane. We find that the field configuration in our previous model that reproduces the polarized synchrotron is not compatible with the Wilkinson Microwave Anisotropy Probe (WMAP) 94 GHz polarized emission data. In particular, the high degree of dust polarization in the outer Galaxy (90 degrees < l < 270 degrees) 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 polarization 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 Galactocentric radius. We discuss the implications for how the spiral-arm compression affects the various components of the magnetized interstellar medium, but conclude that improved data such as that expected from the Planck satellite will be required for a thorough analysis.