KIPAC Tea Talk: Supermassive black hole merger predictions for pulsar timing arrays from cosmological simulations

Abstract: The co-evolution of supermassive black holes (SMBHs) and their host galaxies is well established within ΛCDM cosmology. The repeated mergers, accretion, and feedback that conspire to regulate this process can be studied in large-scale cosmological simulations, such as Illustris, FABLE, MillenniumTNG and Flamingo. These simulations are also essential for predicting SMBH merger rates and the implications for future gravitational wave observatories such as LISA and pulsar timing arrays (PTAs). In this talk, I will examine key limitations of current large-scale simulations, focusing on how treatments used to follow SMBH dynamics, together with limited mass and spatial resolution can lead to premature SMBH mergers. Using the FABLE simulation, I will demonstrate how introducing physically motivated ‘macrophysical’ merger delays to correct for numerical artefacts modifies SMBH merger rates and, consequently, the predicted GW signals. I will then compare these refined predictions with the latest PTA constraints on the stochastic GW background (GWB), showing that while simulated amplitudes typically fall slightly below observed values, there is no statistically significant tension with most of our models. I will also discuss how implications from electromagnetic observations, especially the population of ‘overmassive black holes’ detected by JWST, together with astrophysical uncertainties in the modelling of the SMBH population, leave sufficient flexibility to increase the predicted GWB amplitude, potentially bringing simulations into closer agreement with current PTA data. Finally, I will present a convergence test demonstrating that the absence of rare, massive systems in these cosmological volumes does not significantly affect the predicted GWB.