Published Article: Caught in the Act: Detections of recoiling supermassive black holes from simulations
The coalescence of two spinning supermassive black holes (SMBHs) results in the merged SMBH being kicked out from the centre of the galaxy surrounded by a cluster of stars - a black hole recoil cluster (BRC). We assess the detectability of this phenomenon for large-scale extragalactic surveys such as Euclid, from which targeted follow-up kinematic observations can be done to confirm the presence of an off-centred SMBH. A significantly-increased velocity dispersion coinciding with the BRC, detectable with upcoming Extrememly Large Telescope instruments such as MICADO, offer our best chances of confirming these objects. We use Gaussian process regression and transformation sampling to predict that there should be approximately 8000 such BRCs detectable out to redshift 0.6.
Abstract:
We study the detectability of supermassive black holes (SMBHs) with masses of \((M_\bullet > 10^{9}\,\mathrm{M}_\odot)\) displaced by gravitational wave recoil kicks (\(v_\mathrm{kick} = 0–2000\) km/s) 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 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 \(10^{6} < M_\star < 10^{7}\,\mathrm{M}_\odot\) 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 \(\sigma \sim 600\) km/s 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.