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

Abstract

The Fan Region is one of the dominant features in the polarized radio sky, long thought to be a local (distance less than or similar to 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 H alpha and polarized radio continuum intensity from 0.408 to 353 GHz. The high-frequency (> 1 GHz) and low-frequency (less than or similar to 600 MHz) emissions have different morphologies, suggesting a different physical origin. Portions of the 1.5 GHz Fan Region emission are depolarized by approximate to 30 per cent by ionized gas structures in the Perseus Arm, indicating that this fraction of the emission originates greater than or similar to 2 kpc away. We argue for the same conclusion based on the high polarization fraction at 1.5 GHz (approximate to 40 per cent). The Fan Region is offset with respect to the Galactic plane, covering -5 degrees less than or similar to b less than or similar to + 10 degrees; 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 greater than or similar to 1 GHz emission from the Fan Region as a Galactic scale, not purely local, feature.