High-Redshift Merger Model for Low-Frequency Gravitational Wave Background

TL;DR

This paper proposes an 'early merger' model for the gravitational wave background, incorporating high-redshift galaxy mergers, which better explains recent PTA observations and predicts distinctive high-redshift, massive, rapidly evolving binary sources.

Contribution

It introduces an early merger model based on JWST findings, contrasting it with standard models, to explain the observed gravitational wave background and predict unique source characteristics.

Findings

Early merger model predicts lower detection probability for single binaries.

It suggests a 30% chance that the first detectable source is high-redshift and massive.

The model indicates rapid frequency evolution for certain sources.

Abstract

In 2023, the Pulsar Timing Array (PTA) Collaborations announced the discovery of a gravitational wave background (GWB), predominantly attributed to supermassive black hole binary (SMBHB) mergers. However, the detected GWB is several times stronger than the default value expected from galactic observations at low and moderate redshifts. Recent findings by the James Webb Space Telescope (JWST) have unveiled a substantial number of massive, high-redshift galaxies, suggesting more massive SMBHB mergers at these early epochs. Motivated by these findings, we propose an "early merger" model that complements the standard merger statistics by incorporating these early, massive galaxies. We compare the early and standard "late merger" models, which assume peak merger rates in the local Universe, and match both merger models to the currently detected GWB. Our analysis shows that the early merger model has a significantly lower detection probability for single binaries and predicts a ~30% likelihood that the first detectable single source will be highly redshifted and remarkably massive with rapid frequency evolution. In contrast, the late merger model predicts a nearly monochromatic first source at low redshift. The future confirmation of an enhanced population of massive high-redshift galaxies and the detection of fast-evolving binaries would strongly support the early merger model, offering significant insights into the evolution of galaxies and SMBHs.