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arXiv 2407.06396astro-ph.GA
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有史以来对银河系暖电离介质进行的最灵敏射电复合线测量

The Most Sensitive Radio Recombination Line Measurements Ever Made of the Galactic Warm Ionized Medium

T. M. Bania, Dana S. Balser, Trey V. Wenger, Spencer J. Ireland, L. D. Anderson, Matteo Luisi

AI总结:

本研究利用绿岸望远镜对银河系两条视线方向的暖电离介质进行射电复合线观测,通过叠加跃迁获得高灵敏度光谱,发现等离子体电子温度上限低于传统假设,挑战了经典WIM模型。

摘要:

弥散电离气体遍布银河系盘。我们利用Green Bank Telescope(绿岸望远镜)开展射电复合线(RRL)观测,朝向两条银河系视线方向:G20,$(\boldsymbol{\text{ℓ}}, \boldsymbol{\text{b}}) = (20^\boldsymbol{\text{°}}, 0^\boldsymbol{\text{°}})$,以及G45,$(\boldsymbol{\text{ℓ}}, \boldsymbol{\text{b}}) = (45^\boldsymbol{\text{°}}, 0^\boldsymbol{\text{°}})$,探测到了来自该银河系暖电离介质(WIM)的极微弱辐射。我们对4.3-7.1 GHz之间的18个连续Hn$α$跃迁进行叠加,得到${\rm \boldsymbol{\text{⟨Hnα⟩}}}$光谱,其对发射量EM >10 ${\rm \boldsymbol{\text{cm^{-6}pc}}}$的等离子体的RRL辐射具有灵敏度。每条视线方向都有两个高斯型光谱成分,发射量在$\boldsymbol{\text{∼}}$100至$\boldsymbol{\text{∼}}$300 ${\rm \boldsymbol{\text{cm^{-6}pc}}}$之间。由于在负的本地静止标准(LSR)速度下未探测到可检测的RRL辐射,因此辐射等离子体必定位于太阳轨道内侧。如果这些视线方向充满均匀等离子体,那么G20和G45的发射量对应的均方根密度分别为0.15和0.18$\boldsymbol{\text{cm^{-3}}}$。观测到的${\rm \boldsymbol{\text{⟨Hnβ⟩}}}$/${\rm \boldsymbol{\text{⟨Hnα⟩}}}$线比与最强成分的局域热力学平衡(LTE)激发相一致。G20的高速成分具有13.5 km s$^{-1}$的窄线宽,这为等离子体电子温度设定了<4000 K的上限。这与典型的普遍存在的低密度、$\boldsymbol{\text{∼}}$10000 K的WIM等离子体的假设不一致。

英文摘要:

Diffuse ionized gas pervades the disk of the Milky Way. We detect extremely faint emission from this Galactic Warm Ionized Medium (WIM) using the Green Bank Telescope to make radio recombination line (RRL) observations toward two Milky Way sight lines: G20, $(\ell,{\it b}) = (20^\circ, 0^\circ)$, and G45, $(\ell,{\it b}) = (45^\circ, 0^\circ)$. We stack 18 consecutive Hn$α$ transitions between 4.3-7.1 GHz to derive ${\rm \langle Hnα\rangle}$ spectra that are sensitive to RRL emission from plasmas with emission measures EM >10 ${\rm \,cm^{-6}\,pc}$. Each sight line has two Gaussian shaped spectral components with emission measures that range between $\sim$100 and $\sim$300 ${\rm \,cm^{-6}\,pc}$. Because there is no detectable RRL emission at negative LSR velocities the emitting plasma must be located interior to the Solar orbit. The G20 and G45 emission measures imply RMS densities of 0.15 and 0.18$\,{\rm cm^{-3}}$, respectively, if these sight lines are filled with homogeneous plasma. The observed ${\rm \langle Hnβ\rangle}$/${\rm \langle Hnα\rangle}$ line ratios are consistent with LTE excitation for the strongest components. The high velocity component of G20 has a narrow line width, 13.5 km s$^{-1}$, that sets an upper limit of <4,000 K for the plasma electron temperature. This is inconsistent with the ansatz of a canonically pervasive, low density, $\sim$ 10,000 K WIM plasma.

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