Gravitational waves from intermediate-mass black holes in young clusters
  Massive young clusters (YCs) are expected to host intermediate-mass black
holes (IMBHs) born via runaway collapse. These IMBHs are likely in binaries and
can undergo mergers with other compact objects, such as stellar mass black
holes (BHs) and neutron stars (NSs). We derive the frequency of such mergers
starting from information available in the Local Universe. Mergers of IMBH-NS
and IMBH-BH binaries are sources of gravitational waves (GWs), which might
allow us to reveal the presence of IMBHs. We thus examine their detectability
by current and future GW observatories, both ground- and space-based. In
particular, as representative of different classes of instruments we consider
Initial and Advanced LIGO, the Einstein gravitational-wave Telescope (ET) and
the Laser Interferometer Space Antenna (LISA). We find that IMBH mergers are
unlikely to be detected with instruments operating at the current sensitivity
(Initial LIGO). LISA detections are disfavored by the mass range of IMBH-NS and
IMBH-BH binaries: less than one event per year is expected to be observed by
such instrument. Advanced LIGO is expected to observe a few merger events
involving IMBH binaries in a 1-year long observation. Advanced LIGO is
particularly suited for mergers of relatively light IMBHs (~100 Msun) with
stellar mass BHs. The number of mergers detectable with ET is much larger: tens
(hundreds) of IMBH-NS (IMBH-BH) mergers might be observed per year, according
to the runaway collapse scenario for the formation of IMBHs. We note that our
results are affected by large uncertainties, produced by poor observational
constraints on many of the physical processes involved in this study, such as
the evolution of the YC density with redshift.[abridged]
