Effect of cosmic ray/X-ray ionization on supermassive black hole formation
Effect of cosmic ray/X-ray ionization on supermassive black hole formation
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宇宙射线/X射线电离对超大质量黑洞形成的影响
DOI：
10.1111/j.1365-2966.2011.19229.x
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发表时间：
2011
影响因子：
4.8
通讯作者：
K.Omukai
中科院分区：
文献类型：
--
作者：
K.Inayoshi;K.Omukai
我们研究宇宙射线 (CR) 和 X 射线的外部电离对强远紫外线 (FUV) 辐射下原始云热演化的影响。强烈的 FUV 辐射场可光解 H2 并淬灭其冷却。即使在这样的环境中，维里温度 >rsim104K 的巨大云也可以通过氢莱曼 α 冷却在 8000 K 下等温收缩。该云整体塌陷而没有碎裂，据信在中心形成了一颗超大质量恒星（>rsim105M⊙），最终演变成超大质量黑洞（SMBH）。然而，FUV 源的候选者，包括恒星形成星系，也可能是强 CR 和 X 射线的来源。我们发现外部电离促进了 H2 的产生，并提高了 CR 能量密度 UCR>rsim10−14erg cm-3 或 X 射线强度 JX>rsim10−24erg s-1cm−2sr−1Hz−1 在 1 keV 时形成 SMBH 所需的阈值 FUV 强度 Jcrit 。在高 CR（分别为 X 射线）极限中，临界 FUV 通量随着 Jcrit∝UCR1/2(∝JX1/2) 增加。在莱曼极限 (13.6 eV) 的 FUV 强度值相同的情况下，随着 FUV 源 T* 有效温度的降低，H−光解离速率（阈值为 0.755 eV）增加，而 H2 丰度降低。因此，较低的 T* 值会导致较低的临界 FUV 通量 Jcritat 莱曼极限。利用来自附近恒星形成星系的 FUV 和 CR/X 射线强度之间的经验关系，我们发现外部电离效应显着地将 T* 高达 105K 且由质量 ≲100 M⊙ 的恒星组成的源的临界 FUV 通量提高到任何晕中都无法实现的水平。这表明，要诱导SMBH形成，FUV源必须是具有低亮温（T*∼104K）的族II/I星系、具有非常头重脚轻的初始质量函数的族III星系（T*∼105K）或太年轻而无法容纳CR/X射线源的族III星系，例如超新星遗迹或高质量X射线双星。
We study the effects of external ionization by cosmic rays (CRs) and X-rays on the thermal evolution of primordial clouds under strong far-ultraviolet (FUV) radiation. A strong FUV radiation field photodissociates H2and quenches its cooling. Even in such an environment, a massive cloud with virial temperature >rsim104K can contract isothermally at 8000 K by hydrogen Lyman α cooling. This cloud collapses monolithically without fragmentation, and a supermassive star (>rsim105M⊙) is believed to form at the centre, which eventually evolves into a supermassive black hole (SMBH). However, candidates of FUV sources, including star-forming galaxies, are probably sources of strong CRs and X-rays too. We find that external ionization promotes H2production and elevates the threshold FUV intensityJcritneeded for SMBH formation for CR energy densityUCR>rsim10−14erg cm-3or X-ray intensityJX>rsim10−24erg s-1cm−2sr−1Hz−1at 1 keV. The critical FUV flux increases asJcrit∝UCR1/2(∝JX1/2) in the high CR (respectively X-ray) limit. With the same value of FUV intensity at the Lyman limit (13.6 eV), the H−photodissociation rate, with a threshold of 0.755 eV, increases and the H2abundance decreases with decreasing effective temperature of the FUV sourcesT*. A lower value ofT*thus results in a lower critical FUV fluxJcritat the Lyman limit. Using an empirical relation between the intensities of FUV and CRs/X-rays from nearby star-forming galaxies, we find that the external ionization effect remarkably enhances the critical FUV flux for sources withT*as high as 105K and composed of stars with mass ≲100 M⊙to a level that is not realized in any halo. This indicates that to induce SMBH formation the FUV sources must be either Population II/I galaxies with low brightness temperature (T*∼ 104K), Population III galaxies (T*∼ 105K) with a very top-heavy initial mass function or Population III galaxies too young to harbour sources of CRs/X-rays, for example supernova remnants or high-mass X-ray binaries.