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

Context. Cosmic rays (CRs) and the Galactic magnetic field (GMF) are fundamental actors in many processes in the Milky Way. The observed interaction product of these actors is Galactic synchrotron emission integrated over the line of sight (LOS). A comparison to simulations can be made with this tracer using existing GMF models and CR density models. This probes the GMF strength and morphology and the CR density.
Aims: Our aim is to provide insight into the Galactic CR density and the distribution and morphology of the GMF strength by exploring and explaining the differences between the simulations and observations of synchrotron intensity.
Methods: 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, that is, the emissivity, for HII regions at different distances, allows us to probe the variation in synchrotron emission not only across the sky but also in the third dimension of distance. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using a number of existing GMF models in conjunction with the Galactic CR modeling code GALPROP, we can simulate these synchrotron emissivities.
Results: We present an updated catalog, compiled from the literature, of low-frequency absorption measurements of HII regions, their distances, and electron temperatures. We report a simulated emissivity that shows a compatible trend for HII regions that are near the observer. However, we observe a systematically increasing synchrotron emissivity for HII regions that are far from the observer, which is not compatible with the values simulated by the GMF models and GALPROP.
Conclusions: Current GMF models plus a GALPROP generated CR density model cannot explain low-frequency absorption measurements. One possibility is that distances to all HII regions catalogued at the kinematic "far" distance are erroneously determined, although this is unlikely since it ignores all evidence for far distances 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.