The Fan Region at 1.5 GHz. I: Polarized synchrotron emission extending beyond the Perseus Arm

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 (GMIMS) and compare them to maps of Hα and polarized radio continuum intensity from 0.408 − 353 GHz. The high-frequency (> 1 GHz) and lowfrequency (. 600 MHz) emission have different morphologies, suggesting a different physical origin. Portions of the 1.5 GHz Fan Region emission are depolarized by ≈ 30% 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%). 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.