Caught in the Act: Detections of Recoiling Supermassive Black Holes from Simulations

We study the detectability of supermassive black holes (SMBHs) with masses of M • ≳ 10 9 M ⊙ displaced by gravitational wave recoil kicks ( v kick = 0–2000 km s −1 ) in simulations of merging massive ( M ⋆ > 10 11 M ⊙ ) early-type galaxies. The used Ketju code combines the gadget-4 fast multiple gravity solver with accurate regularized integration and post-Newtonian corrections (up to PN3.5) around SMBHs. The ejected SMBHs carry clusters of bound stellar material (black hole recoil clusters, BRCs) with masses in the range of 10 6 ≲ M BRC ≲ 10 7 M ⊙ and sizes of several 10 pc. For recoil velocities up to 60% of the galaxy escape velocity, the BRCs are detectable in mock photometric images at a Euclid-like resolution up to redshift z ∼ 1.0. By Monte Carlo sampling the observability for different recoil directions and magnitudes, we predict that in ∼20% of instances the BRCs are photometrically detectable, most likely for kicks with SMBH apocentres less than the galaxy effective radius. BRCs occupy distinct regions in the stellar mass/velocity dispersion versus size relations of known star clusters and galaxies. An enhanced velocity dispersion in excess of σ ∼ 600 km s −1 coinciding with the SMBH position provides the best evidence for an SMBH-hosting stellar system, effectively distinguishing BRCs from other faint stellar systems. BRCs are promising candidates to observe the aftermath of the yet-undetected mergers of the most massive SMBHs and we estimate that up to 8000 BRCs might be observable below z ≲ 0.6 with large-scale photometric surveys such as Euclid and upcoming high-resolution imaging and spectroscopy with the Extremely Large Telescope.