光滑环核气盘亚pc分辨率模拟中LISA大质量黑洞双星的不规则合并路径
The erratic path to coalescence of LISA massive black hole binaries in sub-pc resolution simulations of smooth circumnuclear gas disks
摘要：
我们报告的高分辨率模拟，探索轨道衰变的大质量黑洞（MBH）与$10^5$和$10^7 M_{\odot}$嵌入在一个环核气体盘（CND）之间的质量对。采用绝热状态方程，具有一系列绝热指数，保持平稳流动。在这个质量范围内的MBH之间的合并将被即将到来的激光干涉仪空间天线（LISA）检测到。从CND尺度（$100$ ~pc）到$0.1$- $0.01$ ~pc的轨道演化，在此轨道上可以形成环双星盘（CBD）。衰减是不稳定的，并且强烈地依赖于盘内的气流，这最终决定了下沉MBH所经历的净扭矩。总的来说，我们可以确定三个不同的演化阶段：（i）最初的缓慢衰减，导致轨道角动量没有显着变化，导致一些圆化;（ii）快速迁移阶段，轨道角动量迅速减少;（iii）最终，非常缓慢的衰减阶段，轨道角动量甚至可以增加，CBD可以形成。快速迁移阶段是由于圆盘驱动的扭矩，主要来自次级MBH的共轨区域，距离为1- 3 Hill半径。我们发现与原行星盘中大质量行星的快速III型迁移有很强的相似之处。只有当衰变率变得足够小，使其有足够的时间在主MBH周围雕刻一个空腔时，CBD才会形成，尺度为$\lesssim 1$ ~pc;当这种情况发生时，MBH分离在我们的高分辨率运行中几乎停止。我们建议一个经验修改的缺口开放标准，考虑到这种时间尺度的影响，以及其他偏离标准的假设在文献中。[删节]
英文摘要：
We report on high-resolution simulations that explore the orbital decay of massive black hole (MBH) pairs with masses between $10^5$ and $10^7 M_{\odot}$ embedded in a circumnuclear gas disk (CND). An adiabatic equation of state is adopted, with a range of adiabatic indices, which maintains a smooth flow. Mergers between MBHs in this mass range would be detectable by the upcoming Laser Inteferometer Space Antenna (LISA). The orbital evolution is followed from the CND scale ($100$~pc) down to separations of $0.1$--$0.01$~pc at which a circumbinary disk (CBD) could form. The decay is erratic and strongly dependent on the gas flow within the disk, that ultimately determines the net torques experienced by the sinking MBH. Overall, we can identify three different evolutionary stages: (i) an initially slow decay that leads to no significant change in the orbital angular momentum, resulting in some circularization; (ii) a fast migration phase in which the orbital angular momentum decreases rapidly; and (iii) a final, very slow decay phase, in which orbital angular momentum can even increase, and a CBD can form. The fast migration phase owes to disk-driven torques originating primarily from the co-orbital region of the secondary MBH, at a distance of 1--3 Hill radii. We find strong analogies with fast Type III migration for massive planets in protoplanetary disks. The CBD forms only when the decay rate becomes small enough to allow it enough time to carve a cavity around the primary MBH, at scales $\lesssim 1$~pc; when this happens, the MBH separation nearly stalls in our higher-resolution run. We suggest an empirically modified gap-opening criterion that takes into account such timescale effects as well as other deviations from standard assumptions made in the literature. [Abriged]
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