Revisiting the Distance to Radio Loops I and IV Using Gaia and Radio/Optical Polarization Data

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 toward Loop I of 112 ± 17 pc, 135 ± 20 pc, and <105 pc. Our results strongly suggest that the polarized synchrotron emission toward the North Polar Spur at b > 30° 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 toward part of Loop IV (180 ± 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.

North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?

“…If proven, the existence of such an association would definitely close the controversy and favor a local LNPS. As a matter of fact, these high latitude dust or gas structures are all located at short distances, as demonstrated by various works on gas absorption [37], stellar polarization (e.g., [32], and 3D extinction maps ( [52], also see the b +60 • dust cloud in Figure 2 or other high latitude nearby (100-200 pc) faint structures in Figure 7 (right)). However, the association between gas and dust on one side, and X-ray, gamma-ray and synchrotron structures on the other side, has never been proven.…”

“…Observations directly or indirectly related to LNPS could fill long chapters, and we restrict this introduction to early multi-wavelength observations and to the main findings of the last decade. Several presentations of Loop I and discussions about its origin have been published, see the very deep analysis by [34] and references therein, and for a very recent discussion see [32]. Loop I was first detected in radio as the most prominent Galactic loop, with the NPS at its bottom ([4, 19, 26, 48]).…”

“…However, in both cases the expansion in the halo is strongly dependent on the halo density, and one may assume that his argument is also valid in the jet-like case. The result of [42], presented by the author as confirmation of a distant origin of the LPNS, was also used in an opposite way: arguing that there is no established reason for such a north-south difference of halo gas density, [32] considered the result as evidence against the distant LNPS.…”

The C-Band All-Sky Survey (C-BASS): Template Fitting of Diffuse Galactic Microwave Emission in the Northern Sky

“…In the top panel of tudes with increasing frequency (implying that the CREs at high latitudes are being reaccelerated) or we are measuring the spectral index from a superposition of different components along the lineof-sight at low Galactic latitudes; an argument that has been cited previously as a way of resolving discrepancies in the distance measurements to Loop I (Panopoulou et al 2021). This is especially interesting since the spectral indices we measure using the 408 MHz data around Loop I agree with those in the literature (e.g., Davies et al 2006), implying it is not simply a product of region selection.…”

Galactic halo magnetic fields and UHECR deflections

“…Lastly, we block out the region associated with the North Polar Spur (NPS). Our motivation here is that there are indications that the higher latitudes of the NPS are originating locally rather than from the Galactic center, based on starlight polarisation observations (Panopoulou et al 2021). In order to remain as impartial as possible for the designation of this region, we adopt a cut for it selected in Wolleben (2007).…”