Caught in the act: detections of recoiling supermassive black holes from simulations
Alexander Rawlings, Peter H. Johansson, Thorsten Naab, Antti Rantala, Jens Thomas, Bianca Neureiter
arXiv:2505.17183v1 Announce Type: new
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 $(v_{rm kick}=0mathrm{-}2000,mathrm{km,s}^{-1})$ 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 $sim20%$ 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 $sigma sim 600,mathrm{km,s}^{-1}$ 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 $zlesssim 0.6$ with large-scale photometric surveys such as Euclid and upcoming high-resolution imaging and spectroscopy with the Extremely Large Telescope.arXiv:2505.17183v1 Announce Type: new
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 $(v_{rm kick}=0mathrm{-}2000,mathrm{km,s}^{-1})$ 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 $sim20%$ 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 $sigma sim 600,mathrm{km,s}^{-1}$ 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 $zlesssim 0.6$ with large-scale photometric surveys such as Euclid and upcoming high-resolution imaging and spectroscopy with the Extremely Large Telescope.