M51中心区电荷交换的光谱模拟
Spectral Modeling of Charge Exchange in the Central Region of M51
摘要：
电荷交换（CX）发射揭示了星系致密、多相、环核区域中性和电离星际介质（ISM）成分之间的重要相互作用。我们使用一个模型，包括一个热和CX组件来描述的高分辨率XMM牛顿/RGS光谱的漫发射在中心区域的M51。CX发射的代表性特征--特别是突出的OVII禁戒线和OVIII Ly $γ$线中的过量发射--可以很好地由该模型解释。结合OVIII和OVII波段的Chandra图像，我们发现软X射线辐射主要是由喷流驱动的外流及其与周围中性物质的相互作用决定的。喷流驱动的外流本身可能是一个$\sim 0.59$ keV的热等离子体，主要是太阳下的丰度。它进入周围的中性气体，并产生显著的CX辐射，占7- 28毫米波段漫射X射线辐射的五分之一。在CX过程中的有效界面面积是一个数量级大于射流驱动流出的几何表面积。由核恒星形成驱动的稀薄外流也可能对扩散热和CX发射都有一小部分贡献。活动星系核（AGN）的光电离和热气体本身的共振散射是不利的，尽管过去活动星系核事件的影响可能不被排除。
英文摘要：
Charge exchange (CX) emission reveals the significant interaction between neutral and ionized interstellar medium (ISM) components of the dense, multiphase, circumnuclear region of a galaxy. We use a model including a thermal and a CX components to describe the high-resolution XMM-Newton/RGS spectrum of the diffuse emission in the central region of M51. Representative signatures of CX emission -- especially the prominent OVII forbidden line and the excess emission in the OVIII Ly$γ$ lines -- can be well explained by the model. Combined with the Chandra images in the OVIII and the OVII bands, we find the soft X-ray emission is dominated by the jet-driven outflow and its interaction with the ambient neutral material. The jet-driven outflow itself is likely a thermal plasma of $\sim 0.59$ keV, with mostly sub-solar abundances. It runs into the ambient neutral gas, and produces significant CX emission that accounts for one-fifth of the diffuse X-ray emission in the 7--28 Å band. The effective interface area in the CX process is one order of magnitude greater than the geometrical surface area of the jet-driven outflow. The tenuous outflow driven by the nuclear star formation may also contribute a small portion to both the diffuse thermal and CX emission. The photoionization by the active galactic nuclei (AGNs) and the resonance scattering by the hot gas itself are disfavored, though the effects from past AGN events may not be ruled out.
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