Faraday tomography with CHIME: the `tadpole' feature G137+7

TL;DR

This paper uses CHIME polarization data to analyze the 'tadpole' Faraday rotation feature G137+7, revealing its structure, physical association, and implications for detecting low-density ionized gas.

Contribution

First polarization maps from CHIME are used to analyze the G137+7 feature, demonstrating Faraday synthesis's effectiveness at 600 MHz for studying low-density ionized gas.

Findings

The 'tadpole' feature consists of a depolarized head and a extended tail with similar polarization angles.

The head is a Faraday rotation feature, not a depolarized ring in total intensity.

Faraday synthesis at ~600 MHz detects low-density ionized gas undetectable by other tracers.

Abstract

A direct consequence of Faraday rotation is that the polarized radio sky does not resemble the total intensity sky at long wavelengths. We analyze G137+7, which is undetectable in total intensity but appears as a depolarization feature. We use the first polarization maps from the Canadian Hydrogen Intensity Mapping Experiment. Our $400-729$ MHz bandwidth and angular resolution, $17'$ to $30'$, allow us to use Faraday synthesis to analyze the polarization structure. In polarized intensity and polarization angle maps, we find a "tail" extending $10^\circ$ from the "head" and designate the combined object the "tadpole". Similar polarization angles, distinct from the background, indicate that the head and tail are physically associated. The head appears as a depolarized ring in single channels, but wideband observations show that it is a Faraday rotation feature. Our investigations of H I and H$\alpha$ find no connections to the tadpole. The tail suggests motion of either the gas or an ionizing star through the ISM; the B2(e) star HD 20336 is a candidate. While the head features a coherent, $\sim -8$ rad m$^2$ Faraday depth, Faraday synthesis also identifies multiple components in both the head and tail. We verify the locations of the components in the spectra using QU fitting. Our results show that $\sim$octave-bandwidth Faraday rotation observations at $\sim 600$ MHz are sensitive to low-density ionized or partially-ionized gas which is undetectable in other tracers.