We present here a self-consistent cosmological zoom-in simulation of a triple supermassive black hole (SMBH) system forming in a complex multiple galaxy merger. The simulation is run with an updated version of our code KETJU, which is able to follow the motion of SMBHs down to separations of tens of Schwarzschild radii while simultaneously modeling the large-scale astrophysical processes in the surrounding galaxies, such as gas cooling, star formation, and stellar and AGN feedback. Our simulation produces initially an SMBH binary system for which the hardening process is interrupted by the late arrival of a third SMBH. The KETJU code is able to accurately model the complex behavior occurring in such a triple SMBH system, including the ejection of one SMBH to a kiloparsec-scale orbit in the galaxy due to strong three-body interactions as well as Lidov–Kozai oscillations suppressed by relativistic precession when the SMBHs are in a hierarchical configuration. One pair of SMBHs merges ∼3 Gyr after the initial galaxy merger, while the remaining binary is at a parsec-scale separation when the simulation ends at redshift z = 0. We also show that KETJU can capture the effects of the SMBH binaries and triplets on the surrounding stellar population, which can affect the binary merger timescales as the stellar density in the system evolves. Our results demonstrate the importance of dynamically resolving the complex behavior of multiple SMBHs in galactic mergers, as such systems cannot be readily modeled using simple orbit-averaged semianalytic models.

我们在此展示了在复杂的多星系合并中形成的三重超大质量黑洞 (SMBH) 系统的自洽宇宙放大模拟。模拟是使用我们代码 KETJU 的更新版本运行的，它能够跟踪 SMBH 的运动到数十个 Schwarzschild 半径的分离，同时模拟周围星系中的大规模天体物理过程，例如气体冷却、恒星形成，以及恒星和 AGN 反馈。我们的模拟最初产生了一个 SMBH 二元系统，其硬化过程因第三个 SMBH 的迟到而中断。KETJU 代码能够准确地模拟这种三重 SMBH 系统中发生的复杂行为，包括由于强烈的三体相互作用以及当 SMBH 处于分层配置时被相对论进动抑制的 Lidov-Kozai 振荡而将一个 SMBH 喷射到星系中的千秒差距轨道。在初始星系合并后，一对 SMBH 合并~3 Gyr，而当模拟以红移结束时，剩余的双星处于秒差距z = 0。我们还表明，KETJU 可以捕获 SMBH 双星和三联体对周围恒星群的影响，随着系统中恒星密度的演变，这会影响双星合并时间尺度。我们的结果证明了动态解决星系合并中多个 SMBH 的复杂行为的重要性，因为使用简单的轨道平均半解析​​模型无法轻松地对此类系统进行建模。