Revisiting the Distance to Radio Loops I and IV Using Gaia and Radio/Optical Polarization Data
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.
Additional Information© 2021. The American Astronomical Society. Received 2021 June 27; revised 2021 September 13; accepted 2021 September 14; published 2021 December 1. We thank Samir Johnson for contributing to discussions in the early stages of this work. We thank Matias Vidal for providing coordinates to spurs and relevant code. We thank Juan Soler for providing helpful suggestions, as well as the anonymous reviewer for a very thorough reading of our manuscript. Support for this work was provided by NASA through NASA Hubble Fellowship grant No. HST-HF2-51444.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. The work at the California Institute of Technology was supported by the National Science Foundation (NSF) awards AST-0607857, AST-1010024, AST-1212217, AST-1616227, and AST-1611547 and by NASA award NNX15AF06G. C.D. acknowledges support from an STFC Consolidated Grant (ST/P000649/1). C.D. would like to thank Caltech for their hospitality during extended visits. Based on observations obtained with Planck, an ESA science mission with instruments and contributions directly funded by ESA Member States, NASA, and Canada. This work has made use of data from the European Space Agency (ESA) mission Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Facilities: WMAP - Wilkinson Microwave Anisotropy Probe, Planck - , Gaia. - Software: astropy (Astropy Collaboration et al. 2018), HEALPix (Górski et al. 2005), healpy (Zonca et al. 2019).
Published - Panopoulou_2021_ApJ_922_210.pdf
Accepted Version - 2106.14267.pdf