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arXiv 1012.0039astro-ph.HEastro-ph.CO
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星系团吸积激波区非平衡电离效应的X射线特征

X-ray Signatures of Non-Equilibrium Ionization Effects in Galaxy Cluster Accretion Shock Regions

Ka-Wah Wong, Craig L. Sarazin, Li Ji

摘要:

星系团外围区域的密度非常低,碰撞的时间尺度非常长。因此,重元素在通过吸积激波后将处于欠电离状态。我们利用一维流体动力学模拟系统地研究了LambdaCDM宇宙学中弛豫团簇的非平衡电离效应。我们发现,非平衡电离效应不依赖于集团质量,但强烈依赖于红移,这可以理解的自相似标度参数。在红移较低的星系团中,这种效应更强。我们详细地给出了一个大质量星系团在低红移时的X射线特征,如表面亮度轮廓和发射线。在一般情况下,软发射(0.3-1.0千电子伏)显着增强由欠电离,增强可以接近一个数量级附近的冲击半径。我们发现最突出的非平衡电离签名是O VII和O VIII线比。非平衡电离和碰撞电离平衡模型的比率在超过激波半径一半的半径处相差一个数量级以上。这些非平衡电离签名是同样强大的模型具有不同的非绝热冲击电子加热效率。我们还计算了未来国际X射线天文台(IXO)对O VII和O VIII谱线的探测能力。根据测得的谱线比,我们得出结论,用IXO上的X射线微量热谱仪(XMS)对中等红移的大质量规则星系团进行~130-380 ksec的曝光将足以为非平衡电离模型提供强有力的检验。

英文摘要:

The densities in the outer regions of clusters of galaxies are very low, and the collisional timescales are very long. As a result, heavy elements will be under-ionized after they have passed through the accretion shock. We have studied systematically the effects of non-equilibrium ionization for relaxed clusters in the LambdaCDM cosmology using one-dimensional hydrodynamic simulations. We found that non-equilibrium ionization effects do not depend on cluster mass but depend strongly on redshift which can be understood by self-similar scaling arguments. The effects are stronger for clusters at lower redshifts. We present X-ray signatures such as surface brightness profiles and emission lines in detail for a massive cluster at low redshift. In general, soft emission (0.3-1.0 keV) is enhanced significantly by under-ionization, and the enhancement can be nearly an order of magnitude near the shock radius. The most prominent non-equilibrium ionization signature we found is the O VII and O VIII line ratio. The ratios for non-equilibrium ionization and collisional ionization equilibrium models are different by more than an order of magnitude at radii beyond half of the shock radius. These non-equilibrium ionization signatures are equally strong for models with different non-adiabatic shock electron heating efficiencies. We have also calculated the detectability of the O VII and O VIII lines with the future International X-ray Observatory (IXO). Depending on the line ratio measured, we conclude that an exposure of ~130-380 ksec on a moderate-redshift, massive regular cluster with the X-ray Microcalorimeter Spectrometer (XMS) on the IXO will be sufficient to provide a strong test for the non-equilibrium ionization model.

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