不管是厚还是薄：法拉第层析成像揭示的磁离子介质朝向附近${\rm H\,{\small II}}$区域Sharpless 2-27的多个分量
Through thick or thin: Multiple components of the magneto-ionic medium towards the nearby ${\rm H\,{\small II}}$ region Sharpless 2-27 revealed by Faraday tomography
摘要：
Sharpless 2-27（Sh 2 -27）是由$ζ$ Oph激发的邻近${\rm H\,{\small II}}$区。我们目前的观测偏振无线电发射从300至480 $\,$ MHz对Sh 2 -27，与帕克斯64 $\,$ m射电望远镜的全球磁离子介质调查的一部分。这些观测的角分辨率为$1.35^{\circ}$，并且数据对大角度尺度上的磁离子结构非常敏感。我们证明，背景偏振发射对Sh 2 -27是完全去极化在我们的观察，使我们能够调查的前景。我们分析法拉第层析成像的结果，沿着165 $\,$ pc路径映射磁化星际介质到Sh 2 -27。在这个方向上的法拉第色散函数在三个法拉第深度处具有峰值。我们认为法拉第厚和薄的模型，发现薄的模型是首选。我们进一步模拟这作为法拉第旋转的扩散同步辐射在本地气泡和两个前景中性云。局域磁泡在这个方向上延伸了80 $\,$ pc，我们发现法拉第深度为$-0.8 \pm 0.4\,$ rad $\,$ m $^{-2}$。这表明磁场远离太阳，强度为$-2.5\pm1.2\,μ$ G。近中性云和远中性云的厚度分别约为30 $\,$ pc，我们发现法拉第深度分别为$-6.6\pm0.6\,$ rad $\,$ m $^{-2}$和$+13.7\pm0.8\,$ rad $\,$ m $^{-2}$。我们估计了近云和远云的视线磁场强度分别为$B_{\parallel, \text{near}} \approx -15\,μ\text{G}$和$B_{\parallel, \text{far}} \approx +30\,μ\text{G}$。我们的研究结果表明，法拉第层析成像可以用来调查的磁离子特性的前景功能在前面的附近${\rm H\,{\small II}}$地区。
英文摘要：
Sharpless 2-27 (Sh2-27) is a nearby ${\rm H\,{\small II}}$ region excited by $ζ$Oph. 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 $1.35^{\circ}$, 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 $-0.8 \pm 0.4\,$rad$\,$m$^{-2}$. This indicates a field directed away from the Sun with a strength of $-2.5\pm1.2\,μ$G. The near and far neutral clouds are each about 30$\,$pc thick, and we find Faraday depths of $-6.6\pm0.6\,$rad$\,$m$^{-2}$ and $+13.7\pm0.8\,$rad$\,$m$^{-2}$, respectively. We estimate that the line-of-sight magnetic strengths in the near and far cloud are $B_{\parallel, \text{near}} \approx -15\,μ\text{G}$ and $B_{\parallel, \text{far}} \approx +30\,μ\text{G}$. Our results demonstrate that Faraday tomography can be used to investigate the magneto-ionic properties of foreground features in front of nearby ${\rm H\,{\small II}}$ regions.
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