Probing the Merger Rates of Supermassive Black Holes and Galaxies with Gravitational Waves

Abstract: The mergers of galaxies and supermassive black holes (SMBHs) are key drivers of galaxy evolution, contributing to the growth of both galaxies and their central black holes. Current projects like Pulsar Timing Arrays (PTAs) and upcoming missions such as the Laser Interferometer Space Antenna (LISA), Taiji, and Tianqin are designed to detect gravitational waves (GWs) emitted by SMBH binaries during their inspiral and merger phases. We investigate the capability to probe the merger rates of SMBHs and their host galaxies by combining current PTA detections and mock GW data for LISA-like detectors, while incorporating observational constraints from the $M_{\bullet}-M_*$ relationship and galaxy stellar mass functions. Our findings highlight the critical role of GW detections with LISA-like detectors in exploring the merger rates of galaxies and SMBHs and the timescale of SMBH mergers. Additionally, incorporating PTA constraints on the stochastic gravitational wave background further refines model parameters and reduces uncertainties. Gravitational wave detections offer an independent method for estimating galaxy merger rates, providing a valuable consistency check against rates derived from galaxy pair observations and cosmological simulations. Furthermore, comparing SMBH mass assembly through mergers with growth via accretion provides key insights into the evolutionary history of SMBHs, with the timescale of SMBH binary mergers playing a significant role in shaping their merger rates and merger mass assembly.