2023ApJ...944..112W

Recent progress with pulsar timing array experiments, especially from the NANOGrav collaboration, indicates that we are on the cusp of detecting significant signals from the inspiral of supermassive black hole (BH) binaries. 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 Two Micron All Sky Survey redshift survey (2MASS) galaxy sample with the more distant Massive and Distant Clusters of WISE Survey (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 ultramassive BHs in clusters at z ∼ 1, rather than nearby galaxies.