Core Scouring Dynamics and Gravitational Wave Consequences: Constraints on Supermassive Black Hole Binary Hardening

TL;DR

This study combines galaxy simulations and gravitational wave data to evaluate supermassive black hole binary evolution, revealing stellar scattering alone cannot explain observed gravitational wave signals, implying gas dynamics are crucial.

Contribution

It introduces a multi-messenger approach using cosmological simulations and gravitational wave observations to constrain black hole binary hardening mechanisms.

Findings

Stellar scattering must be faster than N-body predictions to match galaxy core observations.

Stellar scattering alone cannot account for the gravitational wave background turnover.

Gas dynamics likely play a significant role in black hole binary hardening.

Abstract

In this paper we perform a multi-messenger investigation of the efficiency of stellar scattering in tightening supermassive black hole binaries by jointly comparing models to the observed galaxy stellar core population and to results of nanohertz gravitational wave observations. Our model uses merger trees from the IllustrisTNG cosmological suite of simulations to predict stellar mass deficits in core galaxies. We take into account dynamical friction, stellar scattering, and gravitational wave emission and compare to the observed relation between core mass deficit and galaxy stellar mass. We find that to match observations, binary hardening in the stellar scattering regime must be about 1.6 times faster than N-body experiments suggest. Most importantly we find that, even assuming a full loss-cone, hardening by stellar scattering alone is insufficient to explain the low frequency turnover seen in the gravitational wave background. This strongly suggests that gas-dynamics play an important role in hardening and provides a reason to be optimistic about electromagnetically visible binary AGN.