Gravitational waves and stalled satellites from massive galaxy mergers
  at z <= 1
  We present a model for merger-driven evolution of the mass function for
massive galaxies and their central supermassive black holes at late times. We
discuss the current observational evidence in favor of merger-driven massive
galaxy evolution during this epoch, and demonstrate that the observed evolution
of the mass function can be reproduced by evolving an initial mass function
under the assumption of negligible star formation. We calculate the stochastic
gravitational wave signal from the resulting black-hole binary mergers in the
low redshift universe (z <= 1) implied by this model, and find that this
population has a signal-to-noise ratio as much as ~5x larger than previous
estimates for pulsar timing arrays, with an expectation value for the
characteristic strain h_c (f=1 yr^{-1}) = 4.1 x 10^{-15} that may already be in
tension with observational constraints, and a {2-sigma, 3-sigma} lower limit
within this model of h_c (f=1 yr^{-1}) = {1.1 x 10^{-15}, 6.8 x 10^{-16}}. The
strength of this signal is sufficient to make it detectable with high
probability under conservative assumptions within the next several years, if
the principle assumption of merger-driven galaxy evolution since z = 1 holds
true. For cases where a galaxy merger fails to lead to a black hole merger, we
estimate the probability for a given number of satellite unmerged black holes
to remain within a massive host galaxy, and interpret the result in light of
ULX observations. In particular, we find that the brightest cluster galaxies
should have 1-2 such sources with luminosities above 10^{39} erg/s, which is
consistent with the statistics of observed ULXs.
