Swift coalescence of supermassive black holes in cosmological mergers of massive galaxies (1604.00015v2)

Abstract: Supermassive black holes (SMBHs) are ubiquitous in galaxies with a sizable mass. It is expected that a pair of SMBHs originally in the nuclei of two merging galaxies would form a binary and eventually coalesce via a burst of gravitational waves. So far theoretical models and simulations have been unable to predict directly the SMBH merger timescale from ab-initio galaxy formation theory, focusing only on limited phases of the orbital decay of SMBHs under idealized conditions of the galaxy hosts. The predicted SMBH merger timescales are long, of order Gyrs, which could be problematic for future gravitational wave searches. Here we present the first multi-scale $\Lambda$CDM cosmological simulation that follows the orbital decay of a pair of SMBHs in a merger of two typical massive galaxies at $z\sim3$, all the way to the final coalescence driven by gravitational wave emission. The two SMBHs, with masses $\sim10^{8}$ M$_{\odot}$, settle quickly in the nucleus of the merger remnant. The remnant is triaxial and extremely dense due to the dissipative nature of the merger and the intrinsic compactness of galaxies at high redshift. Such properties naturally allow a very efficient hardening of the SMBH binary. The SMBH merger occurs in only $\sim10$ Myr after the galactic cores have merged, which is two orders of magnitude smaller than the Hubble time.

Swift Coalescence of Supermassive Black Holes in Cosmological Mergers

The paper entitled "Swift coalescence of supermassive black holes in cosmological mergers of massive galaxies" presents a groundbreaking effort in simulating the dynamics of supermassive black hole (SMBH) binaries within the framework of $\Lambda$CDM cosmology. The authors employ a sophisticated multi-scale simulation approach to trace the orbital decay and eventual coalescence of SMBHs originating from mergers of massive galaxies at high redshift ($z \sim 3$).

Key Findings

1. Simulation Methodology:
   • The paper utilizes a state-of-the-art cosmological hydrodynamical simulation, further refined by re-sampling at higher resolution, to capture the galaxy merger dynamics. Notably, the simulation tracks the SMBHs from kiloparsec-scale separations down to merger due to gravitational wave emission.
   • The simulation encompasses 9 million stellar particles, revealing intricate details about the gravitational environment influencing SMBH dynamics.
2. Rapid Coalescence:
   • Contrary to prior studies predicting SMBH coalescence timescales of the order of a Gyr, this research demonstrates a merger completion in approximately 10Myr post galactic core merger—a duration markedly short considering cosmological timescales.
   • This acceleration in coalescence is attributed to the dense, triaxial structure of the galaxy merger remnants, which enhances the hardening of the SMBH binary.
3. Implications for Gravitational Wave Astronomy:
   • Swift SMBH mergers have significant implications for gravitational wave detection projects such as eLISA. The rapid merger timeline supports the optimistic forecasts for the detection of gravitational waves from massive SMBHs in the $10^6 - 10^8$ M$_{\odot}$ range.
   • The research suggests that the dearth of Pulsar Timing Array detections at low redshift might be explained by the prolonged merger timescales in less dense galaxy environments prevalent at recent epochs.

Theoretical and Practical Implications

The results challenge prevailing assumptions about SMBH merger dynamics, particularly the notion that dense stellar environments and triaxial potential significantly expedite the process. Given the typical galaxy morphology at high redshifts, swift coalescence may be common, suggesting higher SMBH merger rates during early Universe epochs than previously anticipated.

The methodology offers a template for future simulations, underlining the importance of integrating multi-scale cosmological insights with post-Newtonian corrections for SMBHs dynamics, thereby enhancing the fidelity of predictions concerning astrophysical SMBH mergers.

In sum, this research advances our understanding of galaxy mergers and SMBH binaries, opening avenues for detailed examinations of gravitational wave sources in cosmological contexts. Future studies may explore variations in galactic properties and other environmental factors influencing SMBH dynamics to ascertain the broader applicability of these findings across different cosmic epochs and galaxy types.