Accelerated orbital decay of supermassive black hole binaries in merging nuclear star clusters

Abstract

The coalescence of supermassive black holes (SMBHs) should generate the strongest sources of gravitational waves (GWs) in the Universe. However, the dynamics of their coalescence is the subject of much debate. In this study, we use a suite of N-body simulations to follow the merger of two nuclear star clusters (NSCs), each hosting a SMBH in their centre. We find that the presence of distinct star clusters around each SMBH has important consequences for the dynamical evolution of the SMBH binary: (i) The separation between the SMBHs decreases by a few orders of magnitude in the first few Myrs by the combined effects of dynamical friction and a drag force caused by tidally stripped stars. In fact, this is a significant speedup for equal mass ratio binaries, and becomes extreme for unequal mass ratios, e.g. 1:10 or 1:100, which traditional dynamical friction alone would not permit to bind. (ii) The subsequent binary hardening is driven by the gravitational slingshots between the SMBH binary and stars, and also depends on the mass ratio between the SMBHs. Thus, with this additional drag force, we find that all SMBHs in our suite coalesce within a Hubble time. Given that about 50% of Milky Way sized galaxies host NSCs, our results are encouraging for upcoming GW observations with the Laser Interferometer Space Antenna -LISA -which will detect SMBH coalescence in the 10 4 − 10 7 M mass range.

Ejection of supermassive black holes and implications for merger rates in fuzzy dark matter haloes

“…The significance of the 100pc scale also comes from the fact that it is about an order of magnitude larger than the combined sphere of influence of SMBHs of the relevant masses. Current simulations show that, starting at separations a few times larger than the latter scale, a combination of few-body scattering and overlap of the nuclear clusters surrounding the black holes result in markedly accelerated decrease in separation, leading to a bound binary system (Khan et al 2016(Khan et al , 2018Ogiya et al 2020). In this context, it becomes apparent that if one keeps the separation above 100pc, hierarchical SMBH growth can be effectively halted by FDM fluctuations when max > ∼ 10 −21 eV, if the effect of baryons is small.…”

Refining the mass estimate for the intermediate-mass black hole candidate in NGC 3319

“…The much-anticipated Laser Interferometer Space Antenna (LISA; Amaro-Seoane et al, 2017) will have a designed observational requirement of detecting the coalescence of unequal mass black hole binaries of total intrinsic mass 10 4 -10 6 M at z < 3. The merging of such black holes (similar to NGC 3319*), each embedded in their nuclear star cluster, should coalesce within a Hubble time due to dynamical friction (Ogiya et al, 2020). LISA and the next generation of gravitational wave observatories should also be able to find IMBHs in Milky Way globular clusters and the Local Volume (Arca-Sedda et al, 2020).…”

Astrophysics with the Laser Interferometer Space Antenna

“…where ρ is the density of the stellar background, a is the binary Keplerian semimajor axis and H ≈ 15-20 is a numerical coefficient weakly dependent on the properties of the binary (mass, mass ratio, and eccentricity; Quinlan 1996, but see Ogiya et al 2020). The equation above shows that the shrinking rate would be constant for fixed ρ and σ; however, the binary surroundings get perturbed by its scouring action, resulting typically in a mildly declining hardening rate (Vasiliev et al, 2015;Bortolas et al, 2018a).…”