A more detailed look at Galactic magnetic field models: using free-free absorption in HII regions (2002.09204v1)

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.

• The paper presents a novel approach by comparing observed free-free absorption measurements with simulated synchrotron emissivities using several GMF models.
• It employs the GALPROP code alongside multiple magnetic field models to investigate the distribution of cosmic ray densities in the Milky Way.
• The findings reveal a systematic increase in synchrotron emissivity for distant HII regions, indicating potential model inadequacies and the need for refined distance measurements.

Overview of Galactic Magnetic Field Models Using Free-Free Absorption in HII Regions

The paper "A more detailed look at Galactic magnetic field models: using free-free absorption in HII regions" by Polderman et al. presents an investigation into the distribution and morphology of Galactic magnetic field (GMF) strength as well as Cosmic Ray (CR) density in the Milky Way. This research leverages low-frequency synchrotron emission observations from HII regions, examining both observational data and simulations based on existing models.

The authors aim to provide a comprehensive insight into Galactic synchrotron emission by analyzing changes between simulated and observed synchrotron intensity using GMF and CR density models. This approach is intended to probe the GMF strength and morphology and assess the Galactic CR density through synchrotron emissivity determinations, utilizing data from HII regions at varying distances from the observer.

Methodology

The paper uses free-free absorption to measure synchrotron intensity through astrophysical phenomena. At low radio frequencies, HII regions act as opaque blocks, absorbing synchrotron radiation. The paper presents a methodology to measure synchrotron emissivity (intensity per unit path length) for each HII region, which helps gauge variations across different line-of-sight (LOS) distances in a more three-dimensional manner.

Key to their method is the use of the GALPROP code to simulate CR density and several GMF models to replicate synchrotron emissivities. By employing a variety of GMF models (e.g., JF12b, Sun10b, and J13b), the authors can compare simulated results with their compiled catalog of HII regions and observations.

Findings

A significant outcome of this paper is the observed deviation between simulated and actual synchrotron emissions. Notably, while simulations align well with emissivities for HII regions closer to us, the dataset points to an unexplained systematic increase in synchrotron emissivity for more distant HII regions. Traditional GMF models combined with GALPROP do not account for this trend, suggesting potential enhancements in synchrotron emissivity in the outer Galaxy or deficits in the inner Galaxy that current models fail to capture.

Moreover, the results indicate that:

• Traditional GMF models are unable to fully replicate observed low-frequency absorption measurements.
• The hypothesized local synchrotron emissivity enhancement near the Galactic plane is consistent across different GMF and CR models.
• Detection biases may play a role in observed discrepancies, particularly for distant HII regions, indicating a possible need for reevaluation of distance determinations.

Implications

The discrepancies highlighted in this research challenge existing GMF and CR models, illustrating the need for enhancements or reconceptualization on how these models account for synchrotron emissions, especially in the outer regions of the Galaxy. This necessitates further investigation into the underlying causes, including potential structural or density variations within the GMF and CR distribution not currently captured by prevailing models.

By identifying these inconsistencies, the authors urge the need for improved accuracy in distance measurements and consideration of potential detection biases that may impact the observed synchrotron emission profiles.

Future Directions

This paper underscores the complexity of modeling Galactic structures and the intense interplay between CR densities and GMF characteristics. Future explorations could include re-assessments of the underlying GMF logic, incorporating more comprehensive measurements across different bands to verify these trends.

As a result, this paper provides a valuable foundation for subsequent work to refine models of the Galaxy's magnetic field and deepens our understanding of the cosmic ray population and its interactions with Galactic magnetic phenomena. Improvements in data collection methods, achieving greater accuracy in distance assessments, and integration of more diverse magnetic field models could provide the necessary resolution to these disparities.