Through thick or thin: multiple components of the magneto-ionic medium towards the nearby H ii region Sharpless 2–27 revealed by Faraday tomography

Abstract

Sharpless 2-27 (Sh2-27) is a nearby H ii region excited by ζOph. We present observations of polarized radio emission from 300 to 480 MHz towards Sh2-27, made with the Parkes 64 m Radio Telescope as part of the Global Magneto-Ionic Medium Survey. These observations have an angular resolution of 1.35 • , and the data are uniquely sensitive to magneto-ionic structure on large angular scales. We demonstrate that background polarized emission towards Sh2-27 is totally depolarized in our observations, allowing us to investigate the foreground. We analyse the results of Faraday tomography, mapping the magnetised interstellar medium along the 165 pc path to Sh2-27. The Faraday dispersion function in this direction has peaks at three Faraday depths. We consider both Faraday thick and thin models for this observation, finding that the thin model is preferred. We further model this as Faraday rotation of diffuse synchrotron emission in the Local Bubble and in two foreground neutral clouds. The Local Bubble extends for 80 pc in this direction, and we find a Faraday depth of −0.8±0.4 rad m −2 . This indicates a field directed away from the Sun with a strength of −2.5 ± 1.2 µG. The near and far neutral clouds are each about 30 pc thick, and we find Faraday depths of −6.6 ± 0.6 rad m −2 and +13.7 ± 0.8 rad m −2 , respectively. We estimate that the line-of-sight magnetic strengths in the near and far cloud are B ,near ≈ −15 µG and B ,far ≈ +30 µG. Our results demonstrate that Faraday tomography can be used to investigate the magneto-ionic properties of foreground features in front of nearby H ii regions.

The correct sense of Faraday rotation

“…(e.g., Sun et al 2015;Dickey et al 2019;Thomson et al 2019;Stein et al 2020). Clearly, the idea was to keep the order of the integration limits from the source to the observer.…”

The Global Magneto-Ionic Medium Survey (GMIMS): The brightest polarized region in the Southern sky at 75cm and its implications for Radio Loop II

“…This picture may start to break down at low radio frequencies ( 100 MHz) due to depolarization from highly ionised components of the ISM (Van Eck et al 2017). In the GMIMS-LBS band, however, it is reasonable to assume that we can detect emission from most of the ISM within the polarization horizon (Thomson et al 2019). We do find, moreover, that a simple Burn slab model does provide a reasonable description of the longest 𝜆 2 data.…”

“…This calls for further investigation of this region using additional ISM tracers. Such comparison would include neutral hydrogen (H , such as in Thomson et al 2018) and three-dimensional dust maps (such as in Van Eck et al 2017;Thomson et al 2019). A possible correspondence between Loop II and an H feature was commented on by Heiles (1989); however, he does not draw any strong conclusions about the relationship between this feature and Loop II.…”

“…As RM synthesis has been applied to every LOS observed, this survey allows for the application of 'Faraday tomography' (e.g. Van Eck et al 2017Thomson et al 2019). Here we use the term 'Faraday tomography' to refer to the mapping of Faraday depths from diffuse emission across the sky (Ferrière 2016).…”

Magnetism Science with the Square Kilometre Array

“…New all-sky surveys of the diffuse polarized emission from the Milky Way with single dish telescopes have been conducted recently (e.g., [132,133]). These surveys provide polarization observations at multiple frequency channels over a broad bandwidth, which allow us to utilize RM synthesis to study the properties of the Galactic magneto-ionic medium [134,135].…”