Formation and Coalescence of Cosmological Supermassive-Black-Hole Binaries in Supermassive-Star Collapse

We study the collapse of rapidly rotating supermassive stars that may have formed in the early Universe. By self-consistently simulating the dynamics from the onset of collapse using three-dimensional general-relativistic hydrodynamics with fully dynamical spacetime evolution, we show that seed perturbations in the progenitor can lead to the formation of a system of two high-spin supermassive black holes, which inspiral and merge under the emission of powerful gravitational radiation that could be observed at redshifts $z\gtrsim 10$ with the DECIGO or Big Bang Observer gravitational-wave observatories, assuming supermassive stars in the mass range ${10}^4–{10}^6{M}_{\odot }$. The remnant is rapidly spinning with dimensionless spin $a^{*}=0.9$. The surrounding accretion disk contains $\sim 10\%$ of the initial mass.

Figure 1

Snapshots of a slice of the equatorial density distribution of model M2G2. Dark colors indicate high density and light colors indicate low density. The logarithmic density color map ranges from ${10}^{-7}{M}^{-2}$ (white) to ${10}^{-3}{M}^{-2}$ (black). In the bottom two panels, the color map is rescaled to the range [${10}^{-8}{M}^{-2}$, ${10}^{-4}{M}^{-2}$] for the sake of presentation. The upper two and the bottom right panels show an extent of $\pm 40M$, whereas the remaining panels show an extent of $\pm 20M$.

Figure 2

Top panel: evolution of the maximum density for all models. Center panel: Spin and mass evolution of all three black-hole horizons of model M2G2. Lower panel: $(\ell ,m)=(2,2)$ spherical harmonic mode of the $+$ polarization of the emitted gravitational radiation rescaled by distance $D$.