Title: 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 < 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 Halpha
and polarized radio continuum intensity from 0.408-353 GHz. The high-frequency
(> 1 GHz) and low-frequency (< 600 MHz) emission have different morphologies,
suggesting a different physical origin. Portions of the 1.5 GHz Fan Region
emission are depolarized by about 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 (about 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.