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 debated 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±17 pc, 122±32 pc, and < 105 pc. Our results strongly suggest that the polarized synchrotron emission towards 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 towards part of Loop IV (177±17 pc) and find strong 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 [25], stellar polarization (e.g., [5], and 3D extinction maps ( [24], 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 [4] and references therein, and for a very recent discussion see [5]. Loop I was first detected in radio as the most prominent Galactic loop, with the NPS at its bottom ( [1,[6][7][8]).…”

A Unified Model for the Fan Region and the North Polar Spur: A bundle of filaments in the Local Galaxy

“…1. Its alignment with polarized emission from nearby dust grains (Skalidis & Pelgrims 2019;Das et al 2020;Panopoulou et al 2021).…”

“…4. Panopoulou et al (2021) analyze a combination of stellar polarization, polarized synchrotron radio emission, and polarized thermal dust emission to show a significant alignment of the relative angles from these tracers concluding that the NPS is local for b > 30 • .…”

The width of Herschel filaments varies with distance

“…Using the dustmaps python package, we queried the Leike et al (2020) map to obtain the differential optical depth (optical depth per parsec) for each line of sight. We converted the differential optical depth to differential G−band extinction, δA G , following Panopoulou et al (2021). For all sightlines, the differential optical depth shows a prominent peak at a certain distance (Fig.…”