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 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.

Formation of counter-rotating and highly eccentric massive black hole binaries in galaxy mergers

“…A similar orbital plane flip has been observed in previous studies (e.g Mirza et al 2017;Khan et al 2019). However, we note some key differences.…”

“…two galaxies the merger remnant is expected to have some degree of rotation. Previous studies have implied that non-zero streaming motion plays a part in the flipping of the orbital plane of a binary (Holley-Bockelmann & Khan 2015; Mirza et al 2017;Khan et al 2019), showing preferential flipping for binaries embedded in systems with an excess of stars that counter-rotate with the binary. It should be noted that the interpretation of this effect in previous studies has been different, where we consider the rotation of the global potential and not of the relative velocity between the binary SMBH and surrounding matter.…”

“…We conclude by briefly comparing to previous work. Mirza et al (2017) and Khan et al (2019) considered rotating, axisymmetric galaxy models and noted a similar SMBH binary orbital plane flip, but only for initially counter-rotating systems, leading to co-rotating binaries. They attributed this to the preferential ejection of prograde stars by the binary ).…”

“…However, we note some key differences. Mirza et al (2017) and Khan et al (2019) consider the evolution of SMBH binaries in equilibrium axisymmetric galaxy models; they introduce a net stream rotation by flipping the direction of the velocity components of the 𝑁-body particles in their models. They show that during the pairing phase, the eccentricity of the binary dramatically increases for retrograde configurations and the orbital plane flips so that the binary becomes prograde, which suppresses the further eccentricity growth.…”

“…For example, when most stars are counter-rotating with the SMBHs, the binary eccentricity is expected to increase during the hardening phase, while the binary eccentricity decreases in co-rotating distributions (Iwasawa et al 2011;Sesana et al 2011;Holley-Bockelmann & Khan 2015;Rasskazov & Merritt 2017). Recent numerical simulations have also shown that an eccentric binary that is initially retrograde will reverse its sense of rotation (i.e., flip its orbital plane) roughly at the time of binary formation (Mirza et al 2017;Khan et al 2019), from retrograde to prograde. Two distinct processes described in the literature can cause angular momentum flips: local 3-body interaction between the binary SMBH and single stars at small separations which reorients the orbital plane (e.g.…”