Gravitational Waves from Black Holes in Merging Ultra-Dwarf Galaxies
  The origin of the black-hole:black-hole mergers discovered through
gravitational waves with for example the LIGO/Virgo collaboration are a
mystery. We investigate the idea that some of these black holes originate from
the centers of extremely low-mass ultra-dwarf galaxies that have merged
together in the distant past at $z>1$. Extrapolating the central black hole to
stellar mass ratio suggests that the black holes in these mergers could arise
from galaxies of masses $\sim 10^{5} - 10^{6}$ M$_{\odot}\,$. We investigate
whether these galaxies merge enough, or too much, to be consistent with the
observed GW rate of $\sim 9.7-101$ Gpc$^{-3}$ yr$^{-1}$ using the latest
LIGO/Virgo results. We show that in the nearby universe the merger rate and
number densities of ultra-dwarf galaxies are too low, by an order or magnitude,
to produce these black hole mergers. However, by considering that the merger
fraction, merger-time scales, and the number densities of low-mass galaxies all
conspire at $z>1-1.5$ to increase the merger rate for these galaxies at higher
redshifts we argue that it is possible that some of the observed GW events
arise from BHs in the centers of low-mass galaxies. The major uncertainty in
this calculation is the dynamical time-scales for black holes in low-mass
galaxies. Our results however suggest a very long BH merger time-scale of 4-7
Gyr, consistent with an extended black hole merger history. Further simulations
are needed to verify this possibility, however our theory can be tested by
searching for host galaxies of gravitational wave events. Results from these
searches would will put limits on dwarf galaxy mergers and/or the presence and
formation mechanisms of black holes through PopIII stars in the lowest mass
galaxies.
