On black hole evolution in active galactic nuclei
On black hole evolution in active galactic nuclei
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DOI：
10.1093/mnras/283.3.854
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发表时间：
1996-12
影响因子：
4.8
通讯作者：
R. Moderski;M. Sikora
中科院分区：
文献类型：
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作者：
R. Moderski;M. Sikora
我们研究了活动星系核（AGN）中超大质量黑洞的演化，在Shakura-Sunyaev吸积盘模型的框架下，考虑了物质对黑洞的吸积，以及通过Blandford-Znajek机制提取黑洞能量.我们发现，对于高吸积率，黑洞自旋迅速从其初始值的平衡值所确定的吸积盘的旋转速度和旋转速度之间的平衡的Blandford-Znajek机制。对于接近爱丁顿速率的吸积率，黑洞的平衡自旋接近最大值，根据“自旋范式”，具有这种自旋的黑洞的类星体应该是射电大声的。平衡自旋被证明随着吸积率的降低而降低。然而，最初快速旋转的黑洞只能在非常小的粘性参数（α < 0.03）下达到亚哈勃时间尺度上的低自旋。黑洞减速的最佳条件是吸积率，在这种情况下，吸积盘的最里面部分仍然由辐射压力控制。这是因为黑洞角动量的电磁提取率与吸积盘中的总压力成正比，而在辐射压力占主导地位的吸积盘中，吸积率与吸积率无关;在吸积盘完全由气体压力占主导地位的较低吸积率下，角动量提取率与.因此，当吸积速率很低时，减速的时间尺度比哈勃时间长得多，除非α = 0.03。我们结合不同的类星体吸积历史讨论了我们的结果。并研究在螺旋星系中的类星体在低活动期是否有更高的吸积率（与椭圆星系中的类星体相比）最终会导致观测到的类星体的射电强-射电静二分法。
We study the evolution of supermassive black holes in active galactic nuclei (AGNs), taking into account both accretion of matter on to the black hole, and extraction of the black hole energy via the Blandford—Znajek mechanism within the framework of the Shakura—Sunyaev accretion disc model. We show that for high accretion rates, the black hole spin evolves quickly from its initial value to the equilibrium value as determined by a balance between the rate of spinning up by an accretion disc and the rate of spinning down by the Blandford—Znajek mechanism. for accretion rates close to the Eddington rate, the equilibrium spin is close to the maximum value for black holes and, according to the ‘spin paradigm’, quasars hosting black holes with such spins should be radio loud.The equilibrium spins are shown to decrease with decreasing accretion rate. However, initially fast-spinning black holes can reach low spins on sub-Hubble time-scales only for very small viscosity parameters, α < 0.03. The optimal conditions for black hole deceleration are for accretion rates , for which innermost parts of an accretion disc are still dominated by radiation pressure. This is because the rate of electromagnetic extraction of black hole angular momentum is proportional to the total pressure in a disc, which in radiation pressure dominated discs does not depend on the accretion rate.For lower accretion rates, , where the accretion discs are totally dominated by gas pressure, the angular momentum extraction rate is proportional to . Therefore, for very low accretion rates, , the time-scale of deceleration becomes much longer than the Hubble time, unless α ≪ 0.03.We discuss our results in the context of different quasar accretion histories, and investigate whether higher accretion rates during low activity periods in quasars hosted by spiral galaxies (as compared to quasars hosted by elliptical galaxies) can eventually lead to the observed radio-loud—radio-quiet dichotomy of quasars.