Revisiting the distance to radio Loops I and IV using Gaia and radio/optical polarization data

TL;DR

This study uses Gaia data and polarization measurements to determine the distances to radio Loops I and IV, revealing their local origin and challenging previous models of their Galactic center origin.

Contribution

It introduces a method combining optical polarization, Gaia distances, and synchrotron data to accurately measure the distances to radio spurs, confirming their local nature.

Findings

Distances to Loop I regions are approximately 112 and 135 parsecs.

Loop IV's part is at about 180 parsecs.

Synchrotron emission is local, not from the Galactic center.

Abstract

Galactic synchrotron emission exhibits large-angular-scale features known as radio spurs and loops. Determining the physical size of these structures is important for understanding the local interstellar structure and for modeling the Galactic magnetic field. However, the distance to these structures is either under debate or entirely unknown. We revisit a classical method of finding the location of radio spurs by comparing optical polarization angles with those of synchrotron emission as a function of distance. We consider three tracers of the magnetic field: stellar polarization, polarized synchrotron radio emission, and polarized thermal dust emission. We employ archival measurements of optical starlight polarization and Gaia distances, and construct a new map of polarized synchrotron emission from WMAP and Planck data. We confirm that synchrotron, dust emission, and stellar polarization angles all show a statistically significant alignment at high Galactic latitude. We obtain distance limits to three regions towards Loop I of 112$\pm$17 pc, 135$\pm$20 pc, and $<105$ pc. Our results strongly suggest that the polarized synchrotron emission towards the North Polar Spur at $b > 30^\circ$ is local. This is consistent with the conclusions of earlier work based on stellar polarization and extinction, but in stark contrast with the Galactic center origin recently revisited on the basis of X-ray data. We also obtain a distance measurement towards part of Loop IV (180$\pm$15 pc) and find evidence that its synchrotron emission arises from chance overlap of structures located at different distances. Future optical polarization surveys will allow the expansion of this analysis to other radio spurs.