Central Cluster Galaxies: A Hotspot for Detectable Gravitational Waves from Black Hole Mergers

TL;DR

This paper models continuous gravitational wave sources from massive black hole mergers in galaxy clusters, predicting their detectability by PTAs and exploring associated electromagnetic signatures, thus advancing understanding of the low-frequency gravitational wave universe.

Contribution

It introduces a detailed simulation of PTA-detectable continuous gravitational waves from MBH mergers in galaxy clusters, highlighting the significance of massive, cluster-center mergers and triple events.

Findings

Detectable CW sources mainly from MBH mergers >10^10 solar masses.

Most detectable events occur in massive galaxies within clusters.

Triple mergers produce high-probability CW signals at specific frequencies.

Abstract

After Pulsar Timing Arrays (PTAs) have announced the evidence for a low-frequency gravitational wave background (GWB), the continuous waves (CWs) are the next anticipated gravitational wave (GW) signals. In this work, we model CW sources detectable by PTAs based on the massive black hole (MBH) merger population in the ASTRID cosmological simulation. We evolve MBH binaries, simulate their GW emissions, and calculate their detection probability (DP) for PTAs. The most detectable CW sources are produced by MBH mergers with masses M_BH > 10^10 solarmass in the lowest frequency bins with f<10 nHz. Remarkably, these mergers occur within massive galaxies with the stellar mass larger than 10^12 solarmass located at the center of galaxy clusters. Particularly striking in ASTRID is a triple merger event, wherein two consecutive mergers occur within 500 Myr interval in the same cluster core, generating high-DP CW signals at ~ 2nHz and ~ 10nHz. We also investigate the electromagnetic (EM) signatures associated with these events: either single or dual active galactic nuclei (AGN) in the massive host galaxies that are undergoing star formation. This research provides new insights into the low-frequency GW sky and informs future multi-messenger searches for PTA CW sources.