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

## TL;DR

This study reveals that the Fan Region's polarized synchrotron emission at 1.5 GHz extends beyond the local neighborhood, indicating a Galactic-scale origin influenced by the spiral magnetic field and Galactic warp.

## Contribution

The paper provides new evidence that the Fan Region's polarized emission is not solely local but extends over 2 kpc, challenging previous assumptions and emphasizing the need for Galactic magnetic field models to include this feature.

## Key findings

- The Fan Region's high-frequency emission is partially depolarized by the Perseus Arm structures.
- Approximately 40% polarization fraction at 1.5 GHz suggests a distant origin.
- The Fan Region's offset from the Galactic plane is linked to the Galactic warp.

## 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{\deg} < b < +10{\deg}; 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.