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

We model here the merger histories of the supermassive black hole (SMBH) population in the late stages of a cosmological simulation of a ~ 2 × 10¹³ M _⊙ galaxy group. The gravitational dynamics around the several tens of SMBHs (M _• > 7.5 × 10⁷ M _⊙) hosted by the galaxies in the group is computed at high accuracy using regularized integration with the KETJU code. The 11 SMBHs that form binaries and a hierarchical triplet eventually merge after hardening through dynamical friction, stellar scattering, and gravitational wave (GW) emission. The binaries form at eccentricities of e ~ 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 _•-σ _⋆ 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 _•-σ _⋆ 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 ~4 Gyr time span simulated with KETJU.