Compact-object Mergers in the Galactic Center: Evolution in Triaxial Clusters

Abstract

There is significant observational evidence that a large fraction of galactic centers, including those in the Milky Way and M31, host a supermassive black hole (SMBH) embedded in a triaxial nuclear star cluster. In this work, we study the secular orbital evolution of binaries in these environments and characterize the regions and morphological properties of nuclear star clusters that lead to gravitational wave mergers and/or tidal captures. We show that even a modest level of triaxiality in the density distribution of a cluster (an ellipsoid with axis ratios of 0.7 and 0.95) dramatically enhances the merger rates in the central parsecs of the Galaxy by a factor of up to ∼10-30 relative to a spherical density distribution. Moreover, we show that the merger fraction of binaries with semimajor axes in the range 10-100 au remains above 10% for the entire central parsec of the cluster, reaching values close to unity at a distance of ∼0.2-0.4 pc from the SMBH. We understand this large merger efficiency in terms of two distinct mechanisms: (i) eccentricity oscillations driven by the dominant axisymmetric part of the cluster potential that are enhanced by the slow modulation of a binary's angular momentum from the triaxial contribution, similar to the wellknown octupole-level dynamics in three-body systems; and (ii) chaotic diffusion of eccentricities arising when the nodal precession timescale of a binary's orbit about the SMBH becomes comparable to its characteristic secular timescale. Overall, our results indicate that galactic centers are significantly more collisional than previously thought, with mergers taking place up to the effective radii of their nuclear star clusters.

The influence of the secular perturbation of an intermediate-mass companion: I. Eccentricity excitation of disk stars at the Galactic center

“…We assume spherical symmetry for the Galactic bulge . Although tri-axial mass distribution in these components can lead to precessions with long term implications on the secular evolution of single and binary stars' orbits (Petrovich & Antonini 2017;Bub & Petrovich 2020), these effects may be negligible in comparison with the IMC's secular perturbation. We introduce a set of mass and length scaling by factors of 4 × 10 6 and 10 3 respectively (see Table 1) to highlight the similarity between the Galactic center and the Solar system.…”

Nuclear star clusters

“…In NSCs lacking a central massive BH, stellar mass BHs will mass segregate to the center of the cluster and then can form binaries and harden through threebody interactions (e.g., Antonini and Rasio 2016;Choksi et al 2019). The presence of a massive BH also enables stellar mass BH binaries to harden through Kozai-Lidov interactions with the massive BH (e.g., Antonini and Perets 2012;Zhang et al 2019;Bub and Petrovich 2020), or via gas torques and/or hardening encounters with single stars in AGN disks (McKernan et al 2018;Leigh et al 2018). The presence of gas can even facilitate the formation of compact object mergers via dissipative effects (Secunda et al 2019).…”

On the Phase-mixed Eccentricity and Inclination Distributions of Wide Binaries in the Galaxy

“…Heisler & Tremaine (1986) showed that similar secular behavior arises when a wide binary (in their case consisting of the Sun and an Oort comet) is perturbed by the Galactic tide. More recently, Hamilton & Rafikov (2019a, 2019b generalized the LK and Heisler & Tremaine (1986) studies to cover any binary in any axisymmetric potential (see also Brasser et al 2006;Mikkola & Nurmi 2006;Petrovich & Antonini 2017; a further extension to triaxial potentials was studied by Bub & Petrovich 2020). They derived an effective secular Hamiltonian H Γ that encompasses all information about the external potential and the binary's barycentric orbit around that potential in a dimensionless number Γ.…”