Dynamical Processes and Structural Transformations of Supermassive Black Holes in Galaxy Mergers : A Simulation Study

Abstract

Supermassive black holes (SMBHs) are commonly found at the centers of galaxies and exhibit strong correlations with their host galaxies, indicating a close relationship between the growth of SMBHs and galaxy evolution. According to the hierarchical galaxy formation model, this correlation suggests that SMBH binaries must merge efficiently, facilitated by processes such as dynamical friction, interactions with stars and gas, and eventually gravitational wave emission. If SMBH binaries can merge within a time frame shorter than the Hubble time, they could become significant sources of gravitational waves for detection. However, it has been proposed that SMBH binaries might stall at a separation of around 1 parsec, a phenomenon known as the Final Parsec Problem (FPP). This study uses N-body simulations to explore an improved model for the orbital decay of SMBHs driven by dynamical friction. By running a large number of simulations, the findings show that the FPP does not occur in galaxies formed through mergers. The non-spherical shape of the merger remnants ensures a continuous supply of stars for the SMBH binary to interact with, allowing it to continue shrinking its orbit. As the binary progresses toward merging, it ejects stars of multiple times its mass in stars, leading to the formation of low-density cores at the centers of massive elliptical galaxies. These results support the idea that SMBH mergers are frequent in a cosmological context, suggesting that SMBH binaries are likely sources of gravitational waves both at low and high redshifts.