2017MNRAS.467.4631H

The Fan Region is one of the dominant features in the polarized radio sky, long thought to be a local (distance <=500 pc) synchrotron feature. We present 1.3-1.8 GHz polarized radio continuum observations of the region from the Global Magneto-Ionic Medium Survey and compare them to maps of Hα and polarized radio continuum intensity from 0.408 to 353 GHz. The high-frequency (>1 GHz) and low-frequency (<=600 MHz) emissions have different morphologies, suggesting a different physical origin. Portions of the 1.5 GHz Fan Region emission are depolarized by ≃30 per cent by ionized gas structures in the Perseus Arm, indicating that this fraction of the emission originates >=2 kpc away. We argue for the same conclusion based on the high polarization fraction at 1.5 GHz (≃40 per cent). The Fan Region is offset with respect to the Galactic plane, covering -5° <= b <= +10°; we attribute this offset to the warp in the outer Galaxy. We discuss origins of the polarized emission, including the spiral Galactic magnetic field. This idea is a plausible contributing factor although no model to date readily reproduces all of the observations. We conclude that models of the Galactic magnetic field should account for the >=1 GHz emission from the Fan Region as a Galactic scale, not purely local, feature.