While most of the compact-binary mergers detected by LIGO and Virgo are expected to consist of first-generation black holes formed from the collapse of stars, others might instead be of second (or higher) generation, containing the remnants of previous black-hole mergers. We review theoretical findings, astrophysical modelling and current gravitational-wave evidence of hierarchical stellar-mass black-hole mergers. Such a subpopulation of hierarchically assembled black holes presents distinctive gravitational-wave signatures, namely higher masses, possibly within the pair-instability mass gap, and dimensionless spins clustered at the characteristic value of ~0.7. To produce hierarchical mergers, astrophysical environments need to overcome the relativistic recoils imparted to black-hole merger remnants, a condition that prefers hosts with escape speeds of ≳100 km s⁻¹. Promising locations for efficient production of hierarchical mergers include nuclear star clusters and accretion disks surrounding active galactic nuclei, though environments that are less efficient at retaining merger products such as globular clusters may still contribute significantly to the detectable population of repeated mergers. While GW190521 is the single most promising hierarchical-merger candidate to date, constraints from large population analyses are becoming increasingly more powerful.

虽然 LIGO 和 Virgo 探测到的大多数致密双星合并预计由恒星坍缩形成的第一代黑洞组成，但其他可能是第二代（或更高）的黑洞，包含先前黑洞的残余合并。我们回顾了分层恒星质量黑洞合并的理论发现、天体物理模型和当前的引力波证据。这种分层组装的黑洞亚群呈现出独特的引力波特征，即更高的质量，可能在对不稳定性质量间隙内，以及聚集在~0.7 特征值的无量纲自旋。为了产生分层合并，天体物理环境需要克服赋予黑洞合并残余物的相对论反冲，≳ 100 km s ⁻¹。有效产生分层合并的有希望的位置包括核星团和活动星系核周围的吸积盘，尽管在保留合并产物（如球状星团）方面效率较低的环境可能仍然对可检测到的重复合并数量做出重大贡献。虽然 GW190521 是迄今为止最有希望的分层合并候选者，但来自大量人口分析的限制正变得越来越强大。