Kavli Affiliate: Pau Amaro Seoane
| First 5 Authors: Wei Hao, M. B. N. Kouwenhoven, Rainer Spurzem, Pau Amaro Seoane, Rosemary A. Mardling
| Summary:
Supermassive black holes (SMBHs) play an important role in galaxy evolution.
Binary and triple SMBHs can form after galaxy mergers. A third SMBH may
accelerate the SMBH merging process, possibly through the Kozai mechanism. We
use N -body simulations to analyze oscillations in the orbital elements of
hierarchical triple SMBHs with surrounding star clusters in galaxy centers. We
find that SMBH triples spend only a small fraction of time in the hierarchical
merger phase (i.e., a binary SMBH with a distant third SMBH perturber). Most of
the time, the enclosed stellar mass within the orbits of the innermost or the
outermost SMBH is comparable to the SMBH masses, indicating that the influence
of the surrounding stellar population cannot be ignored. We search for
Eccentric Kozai-Lidov (EKL) oscillations for which (i) the eccentricity of the
inner binary and inclination are both oscillate and are anti-phase or in-phase
and (ii) the oscillation period is consistent with EKL timescale. We find that
EKL oscillations are short-lived and rare: the triple SMBH spends around 3% of
its time in this phase over the ensemble of simulations, reaching around 8% in
the best-case scenario. This suggests that the role of the EKL mechanism in
accelerating the SMBH merger process may have been overestimated in previous
studies. We follow-up with three-body simulations, using initial conditions
extracted from the simulation, and the result can to some extent repeat the
observed EKL-like oscillations. This comparison provides clues about why those
EKL oscillations with perturbing stars are short-lived.
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