[论文解读] M82 - A radio continuum and polarisation study II. Polarisation and rotation measures

B. Adebahr, M. Krause+3 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Oct 11, 2017

Astrophysics and Star Formation Studies参考文献 69被引用 8

一句话总结

本研究利用多波长射电偏振与RM合成技术，调查星暴星系M82中的磁场结构。研究揭示了磁化棒状结构通过磁制动驱动7.1 M⊙ yr⁻¹的质量内流，喷流喷流锥中存在螺旋形磁场，以及星系晕中存在大尺度磁场环——表明磁场可在剧烈星暴环境中存活，并将有序磁场输运至星系际空间。

ABSTRACT

The composition and morphology of the interstellar medium in starburst galaxies has been well investigated, but the magnetic field properties are still uncertain. The nearby starburst galaxy M82 provides a unique opportunity to investigate the mechanisms leading to the amplification and reduction of turbulent and regular magnetic fields. Possible scenarios of the contribution of the magnetic field to the star-formation rate are evaluated. Archival data from the VLA and WSRT were combined and re-reduced to cover the wavelength regime between 3cm and 22cm. All observations revealed polarised emission in the inner part of the galaxy, while extended polarised emission up to a distance of 2kpc from the disk was only detected at 18cm and 22cm. The observations hint at a magnetised bar in the inner part of the galaxy. We calculate the mass inflow rate due to magnetic stress of the bar to 7.1 solar masses per year, which can be a significant contribution to the star-formation rate of M82 of approximately 13 solar masses per year. The halo shows polarised emission, which might be the remnant of a regular disk field. Indications for a helical field in the inner part of the outflow cone are provided. The coherence length of the magnetic field in the centre is similar to the size of giant molecular clouds. Using polarisation spectra more evidence for a close coupling of the ionised gas and the magnetic field as well as a two-phase magnetic field topology were found. Electron densities in the halo are similar to the ones found in the Milky Way. The magnetic field morphology is similar to the one in other nearby starburst galaxies with possible large-scale magnetic loops in the halo and a helical magnetic field inside the outflow cones. The special combination of a magnetic bar and a circumnuclear ring are able to significantly raise the star-formation rate in this galaxy by magnetic braking.

研究动机与目标

• 理解磁场在M82中放大并维持恒星形成的作用。
• 确定星暴星系中规则磁场与湍流磁场的起源与形态。
• 评估磁力是否显著贡献于质量内流与恒星形成速率的增强。
• 研究大尺度磁场在剧烈星暴环境中的存活与输运机制。
• 评估磁场对早期星系星系际空间磁化程度的贡献。

提出的方法

• 结合VLA与WSRT在λ3 cm、λ6 cm、λ18 cm和λ22 cm波段的档案数据，进行偏振与旋转量（RM）分析。
• 应用RM合成技术重建法拉第深度结构，并推断磁场几何形态。
• 利用偏振谱识别电离气体与磁场之间的耦合关系，并探测双相磁场拓扑结构。
• 通过偏振结构估算磁场相干长度（50 pc），并与巨大分子云的尺寸进行比较。
• 建模去偏振机制，确定在湍流星暴区域法拉第去偏振占主导地位。
• 从偏振与RM分布的非对称性推断磁化棒状结构的存在，尤其在λ18 cm与λ22 cm波段表现显著。

实验结果

研究问题

• RQ1 M82星系晕与喷流喷流锥中的大尺度磁场形态与起源为何？
• RQ2 磁场在M82中如何促进质量内流与恒星形成速率的增强？
• RQ3 在星暴核心的湍流环境中，规则磁场通过何种机制得以存活？
• RQ4 中心喷流区域是否存在螺旋形磁场结构的证据？
• RQ5 去偏振过程如何影响不同波长下的观测偏振特性？

主要发现

• M82中的磁化棒状结构通过磁应力诱导的质量内流速率达7.1 M⊙ yr⁻¹，超过观测到的分子气体内流速率（3.5 M⊙ yr⁻¹）。
• 在λ18 cm与λ22 cm波段，偏振发射延伸至星系盘面外达2 kpc，表明星系晕中存在大尺度磁场结构。
• 基于延伸区域中法拉第旋转量符号相反及磁场取向的差异，推断中心喷流喷流锥中存在螺旋形磁场。
• 星系中心磁场相干长度估算为50 pc，与巨大分子云的尺寸相当。
• 星系晕中电子密度约为0.009 cm⁻³，与银河系中的观测值一致。
• 在λ3 cm与λ6 cm波段的偏振发射显示，中心磁场方向垂直于星系盘面，延伸至星系晕中0.6 kpc处，反向去偏振比（最高达1.4）表明存在复杂的法拉第效应。