The hierarchical merger model of galaxy formation and evolution predicts that galaxies are assembled through hierarchical mergers along mergers of dark matter halos. A natural consequence of galaxy mergers is the merger of SMBHs from progenitor galaxies. The mechanism or dynamics that drive SMBHs to merge is unknown due to the lack of observational evidence. In this work, we propose to combine the (future) gravitational wave measurements of SMBH binary merger events with the galaxy merger rate distributions from large-scale surveys/cosmological simulations, to infer the delay time of SMBH binary mergers, as a function of binary mass. The future space-based gravitaitonal wave detector LISA or Tianqin/Taiji is optimal to do this work since it will provide us with the population information of the merging binaries (assuming major mergers) across a wide mass range and almost all redshift space, thanks to the large single to noise ratio. We also test different delay time models associated to various dynamical scenarios via the Bayesian model selection method.
Yun Fang obtained her Ph.D degree in July 2020 from the Institute of Theoretical Physics, Chinese Academy of Sciences, supervised by Qing-Guo Huang. She then joined KIAA as a PKING fellow working on the project of gravitational astrophysics, from 2020.09-2022.06, and now she is visiting Perimeter Institute in Canada. Yun Fang is working on the dynamical evolution of compact objects, the generation of gravitational waves, and the multi-messenger and multi-band astrophysics of supermassive black holes.