Coherence of Supermassive Black Hole Binary Demographics with the nHz Stochastic Gravitational-wave Background

Abstract We present a refined estimation of the stochastic gravitational-wave background (SGWB) based on observed dual active galactic nuclei (AGNs) together with AGN X-ray luminosity functions, in light of recent Pulsar Timing Array (PTA) detections of an nHz SGWB. We identify a characteristic luminosity dependence in dual AGN fractions by compiling recent observational data sets, providing crucial constraints on supermassive black hole binary (SMBHB) populations. Our AGN-based model reproduces the current SGWB measurements within PTA observational uncertainties of 2 σ –4 σ , demonstrating consistency between electromagnetic and gravitational-wave observations. These findings establish SMBHBs as the dominant source of the nHz gravitational-wave signal, providing crucial insights into their demographics and evolution.