Signatures of the Many Supermassive Black Hole Mergers in a Cosmologically Forming Massive Early-Type Galaxy

TL;DR

This study models the complex merger histories of supermassive black holes in a massive galaxy group, revealing their dynamical evolution, gravitational wave emissions, and impact on galaxy properties over billions of years.

Contribution

It provides detailed simulations of SMBH mergers in a cosmological context using high-accuracy dynamics, highlighting their gravitational wave signatures and effects on galaxy scaling relations.

Findings

SMBH binaries form with high eccentricities and merge within hundreds of millions of years.

Merger-induced gravitational wave recoil kicks can eject SMBHs, affecting galaxy properties.

Pulsar Timing Arrays and LISA can detect gravitational waves from these SMBH mergers.

Abstract

We model here the merger histories of the supermassive black hole (SMBH) population in the late stages of a cosmological simulation of a $\sim 2 \times 10^{13} M_\odot$ galaxy group. The gravitational dynamics around the several tens of SMBHs ($M_{\bullet} \gtrsim 7.5\times 10^7 M_\odot$) hosted by the galaxies in the group is computed at high accuracy using regularized integration with the KETJU code. The 11 SMBHs which form binaries and hierarchical triplets eventually merge after hardening through dynamical friction, stellar scattering, and gravitational wave (GW) emission. The binaries form at eccentricities of $e \sim 0.3$-$0.9$, with one system evolving to a very high eccentricity of $e = 0.998$, and merge on timescales of a few tens to several hundred megayears. During the simulation the merger-induced GW recoil kicks eject one SMBH remnant from the central host galaxy. This temporarily drives the galaxy off the $M_{\bullet}$-$\sigma_{\star}$ relation, however the galaxy returns to the relation due to subsequent galaxy mergers, which bring in new SMBHs. This showcases a possible mechanism contributing to the observed scatter of the $M_{\bullet}$-$\sigma_{\star}$ relation. Finally, we show that Pulsar Timing Arrays and LISA would be able to detect parts of the GW signals from the SMBH mergers that occur during the $\sim 4\,\mathrm{Gyr}$ time span simulated with KETJU.