Faraday Rotation of Extended Emission as a Probe of the Large-Scale Galactic Magnetic Field

Abstract

The Galactic magnetic field is an integral constituent of the interstellar medium (ISM), and knowledge of its structure is crucial to understanding Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources have been the basis of comprehensive Galactic magnetic field models. Polarised extended emission (XE) is also seen along lines of sight through the Galactic disk, and also displays the effects of Faraday rotation. Our aim is to investigate and understand the relationship between EG and XE RMs near the Galactic plane, and to determine how the XE RMs, a hitherto unused resource, can be used as a probe of the large-scale Galactic magnetic field. We used polarisation data from the Canadian Galactic Plane Survey (CGPS), observed near 1420 MHz with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope. We calculated RMs from a linear fit to the polarisation angles as a function of wavelength squared in four frequency channels, for both the EG sources and the XE. Across the CGPS area, 55 • < < 193 • , −3 • < b < 5 • , the RMs of the XE closely track the RMs of the EG sources, with XE RMs about half the value of EG-source RMs. The exceptions are places where large local HII complexes heavily depolarise more distant emission. We conclude that there is valuable information in the XE RM dataset. The factor of 2 between the two types of RM values is close to that expected from a Burn slab model of the ISM. This result indicates that, at least in the outer Galaxy, the EG and XE sources are likely probing similar depths, and that the Faraday rotating medium and the synchrotron emitting medium have similar variation with galactocentric distance.

CHIME/FRB Discovery of Eight New Repeating Fast Radio Burst Sources

“…The former are being revised, and the latter can often be unreliable at low Galactic latitudes (Ma et al 2019). Ordog et al (2019), based on the revised CGPS data, showed a foreground RM ≈−115±12 rad m −2 in this direction, consistent with a smoothly decreasing trend in RM from longitudes 100°-180°. This suggests the RM of Source 1 arises largely in the Galaxy, leaving only a low contribution for RM host +RM source , certainly far lower than 10 5 rad m −2 , the RM of FRB 121102 (Michilli et al 2018).…”

Nine New Repeating Fast Radio Burst Sources from CHIME/FRB

“…In lowlatitude cases such as this, the first Faraday moment of the emission is unlikely to be an accurate proxy for RM MW . In fact, even idealized scenarios with minimal path length through the foreground and no magnetic field reversals can still produce systematic bias between the first Faraday moment of diffuse, polarized emission, and the true RM MW value (Ordog et al 2019). A larger sample of extragalactic polarized sources, including FRBs, will be helpful in identifying well-behaved regions of the sky where diffuse, polarized emission can be leveraged to better subtract the foreground RM contribution.…”

Revisiting Rotation Measures from the Canadian Galactic Plane Survey: the Magnetic Field in the Disk of the Outer Galaxy

“…This includes both studies of large scale structure in the Galactic magnetic field (Jaffe 2019) as well as studies of magnetism in smaller scale objects (e.g., Tahani et al 2018). Our catalog has already contributed significantly to several such studies (Ordog et al 2019;Ma et al 2020).…”

“…The region of positive RMs around = 75 • , b = +4 • may be associated with the low-longitude side of Cygnus X or may be associated with the positive RMs found at lower longitudes. A study of compact source RMs and a comparison with the RM of extended emission (as in Ordog et al 2019) would contribute to understanding this region.…”

“…They also found that this transition was reflected in the diffuse polarized emission present in the CGPS data. Ordog et al (2019) used a pre-final version of this RM catalog (which was made without the 20% sensitivity threshold and fractional polarization variation tests) to compare with rotation measures derived from the diffuse polarized emission that is also present in the CGPS data. They found a strong correspondence between the diffuse emission RMs and the extragalactic source RMs, except in a few regions (most of the regions that are discussed in Sects.…”