Inward Bound: The incredible journey of massive black holes as they pair and merge; I. The effect of mass ratio in flattened rotating galactic nuclei

Abstract

Understanding how supermassive black holes (SMBHs) pair and merge helps to inform predictions of off-center, dual, and binary AGN, and provides key insights into how SMBHs grow and co-evolve with their galaxy hosts. As the loudest known gravitational wave source, binary SMBH mergers also hold centerstage for the Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave observatory set to launch in 2034. Here, we continue our work to characterize SMBH binary formation and evolution through increasingly more realistic high resolution direct $N$-body simulations, focusing on the effect of SMBH mass ratio, orientation, and eccentricity within a rotating and flattened stellar host. During the dynamical friction phase, we found a prolonged orbital decay for retrograde SMBHs and swift pairing timescales for prograde SMBHs compared to their counterparts in non-rotating models, an effect that becomes more pronounced for smaller mass ratios $M_{\sec }/M_{\mathrm{prim}}=q$. During this pairing phase, the eccentricity dramatically increases for retrograde configurations, but as the binary forms, the orbital plane flips so that it is almost perfectly prograde, which stifles the rapid eccentricity growth. In prograde configurations, SMBH binaries form and remain at comparatively low eccentricities. As in our prior work, we note that the center of mass of a prograde SMBH binary itself settles into an orbit about the center of the galaxy. Since even the initially retrograde binaries flip their orbital plane, we expect few binaries in rotating systems to reside at rest in the dynamic center of the host galaxy, though this effect is smaller as $q$ decreases.

Accepted for publication in MNRAS

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• Circumbinary Disk Accretion into Spinning Black Hole Binaries
• Formation of super-massive black holes in galactic nuclei I: delivering seed intermediate-mass black holes in massive stellar clusters
• Defeating stochasticity: coalescence timescales of massive black holes in galaxy mergers
• Pairing of Massive Black Holes in Merger Galaxies Driven by Dynamical Friction
• Extremely efficient mergers of intermediate mass black hole binaries in nucleated dwarf galaxies
• A Chandra Virgo cluster survey of spiral galaxies. I. Introduction to the survey and a new ULX sample
• Constraints on the Galactic Centre environment from \textit{Gaia} hypervelocity stars III: Insights on a possible companion to Sgr A*
• Examining the effects of dark matter spikes on eccentric intermediate mass ratio inspirals using N-body simulations
• Stellar hardening of massive black hole binaries: the impact of the host rotation
• Astrophysics with the Laser Interferometer Space Antenna
• Causal Discovery in Astrophysics: Unraveling Supermassive Black Hole and Galaxy Coevolution
• Properties of loss cone stars in a cosmological galaxy merger remnant
• Formation of counter-rotating and highly eccentric massive black hole binaries in galaxy mergers
• The evolution of a supermassive binary black hole in an non-spherical nuclear star cluster
• Contaminating Electromagnetic Transients in LISA Gravitational Wave Localization Volumes. I: The Intrinsic Rates