Ultra-Low Frequency Gravitational Waves from Massive Clusters at z~1

TL;DR

This paper suggests that ultra-massive black hole mergers in galaxy clusters at redshift ~1 could produce detectable low-frequency gravitational waves, potentially leading to the first individual source detections by pulsar timing arrays.

Contribution

It introduces the idea that distant galaxy clusters at z~1 may be more significant sources of gravitational waves than nearby galaxies, highlighting a new target for detection.

Findings

Clusters at z~1 can produce larger gravitational wave strains.

Distant clusters may contribute more detectable signals than nearby galaxies.

First detections might come from black hole mergers in clusters at z~1.

Abstract

Recent progress with pulsar timing array experiments, especially from the NANOGrav collaboration indicate that we are on the cusp of detecting significant signals from the inspiral of super-massive black hole binaries (SMBHB). While recent analysis has focused on nearby galaxies as possible sources of the loudest signals, we show that mergers in identified clusters at z~1 can have larger strain amplitudes. We make an estimate comparing the nearby 2MASS redshift survey galaxy sample with the more distant MaDCoWS cluster sample, showing that the latter might be expected to contribute more, and louder, gravitational wave events. Thus the first individual source detections may well be from ultra-massive BH in clusters at z~1, rather than nearby galaxies.