We study the close encounters between two single black holes (BHs) embedded in an AGN disk using a series of global 2D hydrodynamics simulations. We find that when the disk density is sufficiently high, bound BH binaries can be formed by the collision of their circum-single disks. Our analysis demonstrates that, after a BH pair passes the pericenter of their relative trajectory, a gas post-collision drag may slow down the BHs, possibly forcing the two BHs to stay tightly bound. A binary formed by a close encounter can have a compact semi-major axis, large eccentricity, and retrograde orbital angular momentum. We provide a fitting formula that can accurately predict whether a close encounter can form a binary based on the gas mass and the incoming energy of the encounter. This fitting formula can be easily implemented in other long-term simulations that study the dynamical evolution of BHs in AGN disks.

中文翻译：

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活动星系核盘中黑洞双星形成的流体动力学模拟

我们使用一系列全局 2D 流体动力学模拟研究嵌入在 AGN 盘中的两个单个黑洞 (BH) 之间的近距离接触。我们发现，当磁盘密度足够高时，绑定的 BH 双星可以通过其外接单磁盘的碰撞形成。我们的分析表明，在 BH 对通过其相对轨迹的中心周围后，气体碰撞后阻力可能会减慢 BH，可能迫使两个 BH 保持紧密结合。近距离相遇形成的双星可以具有紧凑的半长轴、大的偏心率和逆行的轨道角动量。我们提供了一个拟合公式，可以根据气体质量和相遇的传入能量准确预测近距离相遇是否可以形成双星。