[ABRIDGED] We study supermassive black hole binary mergers driven by angular momentum loss to small-scale gas discs. Such binaries form after major galaxy mergers, but their fate is unclear since hardening through stellar scattering becomes very inefficient at sub-parsec distances. Gas discs may dominate binary dynamics on these scales, and promote mergers. Using numerical simulations, we investigate the evolution of the orbits of binaries embedded within geometrically thin gas discs. Our simulations directly resolve angular momentum transport within the disc, which at the radii of interest is likely dominated by disc self-gravity. We show that the binary decays at a rate which is in good agreement with analytical estimates, while the eccentricity grows. Saturation of eccentricity growth is not observed up to values e > 0.35. Accretion onto the black holes is variable, and is roughly modulated by the binary orbital frequency. Scaling our results, we analytically estimate the maximum rate of binary decay that is possible without fragmentation occuring within the surrounding gas disc, and compare that rate to an estimate of the stellar dynamical hardening rate. For binary masses in the range 10^5 < M/Msun < 10^8 we find that decay due to gas discs may dominate for separations below a ~ 0.01--0.1 pc, where the disc is optically thick. The minimum merger time scale is shorter than the Hubble time for M < 10^7 Msun. This implies that gas discs could commonly attend relatively low mass black hole mergers, and that a significant population of binaries might exist at separations of a few hundredths of a pc, where the combined decay rate is slowest. For more massive binaries, we suggest that scattering of stars formed within a fragmenting gas disc could act as a significant additional sink of binary angular momentum.
在这篇综述中，我们讨论了在遥远的宇宙中星系相互作用和合并的证据，以及合并在形成星系中的作用。观测结果表明，在z~1处，大质量（M> M *）星系参与主要合并的比例约为5-10%。然而，对于最大质量的星系，合并分数急剧增加，直到z~3，合并分数为50+/-20%。使用星系合并过程的N体模型，在各种合并条件下，合并质量比，和视角，合并分数可以转换为合并率，质量吸积率由于合并。对合并速率的简单积分显示，一个典型的z~3的大质量星系在z~3和z~0之间会经历4-5次主要的合并，其中大部分的合并活动和由此产生的质量聚集发生在z > 1.5的时候。
Hydrodynamic simulations of merging clusters of galaxies
我们提出了高分辨率的AP 3 M +SPH模拟合并星系团的结果。我们发现，在合并过程中的核心气体的压缩和冲击，可以导致大的增加，在辐射X射线光度和发射加权温度的集群。在等质量合并过程中，冷却流完全中断，随着团簇核心的碰撞，质量沉积速率降至零。核心气体冷却时间的大幅增加强烈表明，冷却流不会在哈勃时间内从这种合并中恢复。如果子团的质量是主团质量的1/8，则合并也会破坏冷却流。然而，在这种情况下，注入核心气体的熵会迅速辐射出去，冷却流在合并的几个Gyr内重新开始。当子团簇的碰撞参量为500 kpc时，虽然质量沉积速率降低了30%，但并不影响冷却流。最后，我们发现，相等的质量，偏离中心的合并可以有效地混合气体的核心集群，而头合并导致非常小的混合。从子团外层剥离的气体导致主团外层的部分被很好地混合，尽管它们对团核心的气体几乎没有影响。这里所研究的合并没有一个导致ICM在全球范围内混合良好。
