2025ApJ...991...83R

We study the detectability of supermassive black holes (SMBHs) with masses of M_• ≳ 10⁹M_⊙ displaced by gravitational wave recoil kicks (v_kick = 0-2000 km s^–1) in simulations of merging massive (M_⋆ > 10¹¹M_⊙) 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 10⁶ ≲ M_BRC ≲ 10⁷M_⊙ 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.