A more detailed look at Galactic magnetic field models: using free-free absorption in HII regions

Abstract

The observed interaction product of Cosmic Rays (CRs) and Galactic magnetic fields (GMF) is the Galactic synchrotron emission integrated over the line-of-sight (LOS). The GMF strength and morphology and the CR density can be probed by comparing this tracer to simulations using existing GMF models and CR density models. Our aim is to provide insight into these parameters by exploring and explaining the differences between simulations and observations of synchrotron intensity. At low radio frequencies HII regions become opaque due to free-free absorption. Using these HII regions we can measure the synchrotron intensity over a part of the LOS through the Galaxy. The measured intensity per unit path length, i.e. the emissitivity, for HII regions at different distances, will allow probing variation in synchrotron emission in the third dimension of distance. Using a number of existing GMF models in conjunction with the Galactic CR modeling code GALPROP we can simulate these synchrotron emissivities. We present an updated catalog of low-frequency absorption measurements of HII regions compiled from the literature. We report a simulated emissivity that shows a compatible trend for HII regions that are near to the observer. And we observe a systematically increasing synchrotron emissivity for HII regions that are far from the observer, which is not compatible with simulated values. Current GMF and CR density models cannot explain low-frequency absorption measurements. One possibility is that distances to all HII regions at the kinematic 'far' distance are wrong, though this is unlikely as it ignores all evidence in the literature. However, a detection bias due to the nature of this tracer requires us to keep in mind that certain sources may be missed in an observation. The other possibilities are an enhanced emissivity in the outer Galaxy or a diminished emissivity in the inner Galaxy.