Caught in the act: detections of recoiling supermassive black holes from simulations

Abstract

We study the detectability of supermassive black holes (SMBHs) with masses of $M_{\bullet }\gtrsim 10^9{\mathrm{M}}_{\odot }$ displaced by gravitational wave recoil kicks $(v_{\mathrm{kick}}=0-2000{\mathrm{km}\mathrm{s}}^{-1})$ in simulations of merging massive $(M_{\star }>10^{11}{\mathrm{M}}_{\odot })$ early-type galaxies. The used KETJU code combines the GADGET-4 fast multiple gravity solver with accurate regularised 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 $10^6\lesssim M_{\text{BRC}}\lesssim 10^7{\mathrm{M}}_{\odot }$ and sizes of several $10\mathrm{pc}$. 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 $z\sim 1.0$. By Monte Carlo sampling the observability for different recoil directions and magnitudes, we predict that in $\sim 20\%$ 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 vs. size relations of known star clusters and galaxies. An enhanced velocity dispersion in excess of $\ddot{I}ƒ\sim 600{\mathrm{km}\mathrm{s}}^{-1}$ 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 $z\lesssim 0.6$ with large-scale photometric surveys such as Euclid and upcoming high-resolution imaging and spectroscopy with the Extremely Large Telescope.