The direct collapse model for the formation of massive black holes has gained increased support as it provides a natural explanation for the appearance of bright quasars already less than a billion years from the Big Bang. In this paper we review a recent scenario for direct collapse that relies on multi-scale gas inflows initiated by the major merger of massive gas-rich galaxies at z > 6, where gas has already achieved solar composition. Hydrodynamical simulations undertaken to explore our scenario show that supermassive, gravitationally bound compact gaseous disks weighing a billion solar masses, only a few pc in size, form in the nuclei of merger remnants in less than 105 yr. These could later produce a supermassive protostar or supermassive star at their center via various mechanisms. Moreover, we present a new analytical model, based on angular momentum transport in mass-loaded gravitoturbulent disks. This naturally predicts that a nuclear disk accreting at rates exceeding [Formula: see text] yr-1, as seen in the simulations, is stable against fragmentation irrespective of its metallicity. This is at variance with conventional direct collapse scenarios, which require the suppression of gas cooling in metal-free protogalaxies for gas collapse to take place. Such high accretion rates reflect the high free-fall velocities in massive halos appearing only at z < 10, and occur naturally as a result of the efficient angular momentum loss provided by the merger dynamics. We discuss the implications of our scenario on the observed population of high-z quasars and on its evolution to lower redshifts using a semi-analytical galaxy formation model. Finally, we consider the intriguing possibility that the secondary gas inflows in the unstable disks might drive gas to collapse into a supermassive black hole directly via the General Relativistic radial instability. Such dark collapse route could generate gravitational wave emission detectable via the future Laser Interferometer Space Antenna (LISA).

中文翻译：

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通过合并驱动的直接坍缩形成大质量黑洞的途径：综述

形成大质量黑洞的直接坍缩模型得到了越来越多的支持，因为它为距大爆炸不到 10 亿年的明亮类星体的出现提供了自然的解释。在本文中，我们回顾了最近直接坍塌的情景，该情景依赖于由 z > 6 处的大型富含气体的星系的主要合并引发的多尺度气体流入，其中气体已经达到了太阳组成。为探索我们的情景而进行的流体动力学模拟表明，在不到 105 年的时间内，在合并残余物的原子核中形成了重达 10 亿个太阳质量的超大质量、受引力束缚的致密气态盘，其大小只有几 pc。这些后来可以通过各种机制在其中心产生超大质量原恒星或超大质量恒星。此外，我们提出了一个新的分析模型，基于质量加载的重力湍流盘中的角动量传输。这自然地预测，如模拟中所见，以超过 [公式：见文本] yr-1 的速率吸积的核圆盘对于碎裂是稳定的，而与其金属丰度无关。这与传统的直接坍缩情景不同，后者需要抑制无金属原星系中的气体冷却才能发生气体坍塌。如此高的吸积率反映了仅在 z < 10 处出现的大量晕中的高自由落体速度，并且由于合并动力学提供的有效角动量损失而自然发生。我们使用半解析星系形成模型讨论了我们的情景对观测到的高 z 类星体种群及其向低红移演化的影响。最后，我们考虑了一种有趣的可能性，即不稳定圆盘中的二次气体流入可能会通过广义相对论径向不稳定性直接驱动气体坍缩成超大质量黑洞。这种暗坍塌路线可以产生可通过未来激光干涉仪空间天线（LISA）检测到的引力波发射。