Gravitational-wave captures by intermediate-mass black holes in galactic nuclei

Abstract

Intermediate-mass black holes (IMBHs) have not been detected beyond any reasonable doubt, despite their potential role as massive seeds for quasars and sources of tidal disruption events, ultra-luminous X-ray sources, dwarf galaxy feedback, and hypervelocity stars. Gravitational wave (GW) observations can help to find and confirm the existence of IMBHs. Current and upcoming detectors, such as LIGO, Virgo, KAGRA, LISA, ET, and DECIGO promise to identify the full range from stellar-mass to supermassive black holes (SMBHs). In this paper, we address the question of whether IMBHs can produce GWs in galactic nuclei. We consider the possibility that stellar black holes (SBHs) form bound systems and later coalesce with an IMBH through gravitational captures in the dense nucleus. We show that this mechanism is efficient for IMBH masses in the range $\sim 3\times 10^3$ M${}_{\odot }$--$2\times 10^4$ M${}_{\odot }$. We find that the typical distributions of peak frequencies and merger timescales depend mainly on the IMBH mass. In particular, the typical peak frequency is about $0.2$Hz, $0.1$Hz, $0.09$Hz, and $0.05$Hz for $M_{\mathrm{IMBH}}=5\times 10^3$ M${}_{\odot }$, $8\times 10^3$ M${}_{\odot }$, $1\times 10^4$ M${}_{\odot }$, and $2\times 10^4$ M${}_{\odot }$, respectively. Our results show that, at design sensitivity, both DECIGO and ET should be able to detect these IMBH--SBH mergers. Furthermore, most of the mergers will appear eccentric ($e\gtrsim 0.1$), providing an indication of their dynamical origin.

10 pages, 5 figures, accepted by ApJ

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• Confusing Head-On Collisions with Precessing Intermediate-Mass Binary Black Hole Mergers
• The Formation of Intermediate Mass Black Holes in Galactic Nuclei
• Quiescent and active galactic nuclei as factories of merging compact objects in the era of gravitational-wave astronomy
• The Combined Effects of Two-Body Relaxation Processes and the Eccentric Kozai-Lidov Mechanism on the EMRI Rate
• Detecting Kozai-Lidov imprints on the gravitational waves of intermediate-mass black holes in galactic nuclei
• First and second-generation black hole and neutron star mergers in 2+2 quadruples: population statistics
• Demographics of Hierarchical Black Hole Mergers in Dense Star Clusters
• A Stability Timescale for Non-Hierarchical Three-Body Systems
• Constraining a companion of the galactic center black hole, Sgr A*
• Merging Hierarchical Triple Black Hole Systems with Intermediate-mass Black Holes in Population III Star Clusters
• Dynamically Formed Black Hole Binaries: In-Cluster Versus Ejected Mergers
• Calibrating the binary black hole population in nuclear star clusters through tidal disruption events
• LISA Constraints on an Intermediate-Mass Black Hole in the Galactic Centre
• Trigonometric parallax and proper motion of Sagittarius A* measured by VERA using the new broad-band back-end system OCTAVE-DAS
• Identification of Intermediate-mass Black Hole Candidates Among a Sample of Sd Galaxies
• Detection for Intermediate-mass Binary Black Holes in Population III Star Clusters with TianQin