Inferring Mbh-Mbulge Evolution from the Gravitational Wave Background

TL;DR

This study investigates how the evolution of the supermassive black hole to bulge mass relation affects gravitational wave background predictions, revealing that an evolving relation better matches observations without requiring an unrealistically high galaxy mass function.

Contribution

It introduces an evolving Mbh-Mbulge relation into models of the gravitational wave background, improving agreement with observed data compared to static assumptions.

Findings

Evolving Mbh-Mbulge ratio explains higher GWB amplitude.

Static models require an undetected massive galaxy population.

Evolving relation fits GWB data with alpha(z) = alpha_0 (1 + z)^{1.04 +/- 0.5}.

Abstract

We test the impact of an evolving supermassive black hole (SMBH) mass scaling relation (Mbh-Mbulge) on the predictions for the gravitational wave background (GWB). The observed GWB amplitude is 2-3 times higher than predicted by astrophysically informed models which suggests the need to revise the assumptions in those models. We compare a semi-analytic model's ability to reproduce the observed GWB spectrum with a static versus evolving-amplitude Mbh-Mbulge relation. We additionally consider the influence of the choice of galaxy stellar mass function on the modeled GWB spectra. Our models are able to reproduce the GWB amplitude with either a large number density of massive galaxies or a positively evolving Mbh-Mbulge amplitude (i.e., the Mbh / Mbulge ratio was higher in the past). If we assume that the Mbh-Mbulge amplitude does not evolve, our models require a galaxy stellar mass function that implies an undetected population of massive galaxies (Mstellar > 10^11 Msun at z > 1). When the Mbh-Mbulge amplitude is allowed to evolve, we can model the GWB spectrum with all fiducial values and an Mbh-Mbulge amplitude that evolves as alpha(z) = alpha_0 (1 + z)^(1.04 +/- 0.5).