Abstract
Intermediate-mass black holes (IMBHs) have not been detected beyond any
reasonable doubt, despite their important 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 $310^3\,$M$_ødot$--$210^4\,$M$_ødot$. 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 $M_IMBH=5\times
10^3\,$M$_ødot$, $810^3\,$M$_ødot$, $110^4\,$M$_ødot$,
and $210^4\,$M$_ødot$, 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.
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