An estimate of the stochastic gravitational wave background from the
  MassiveBlackII simulation
  A population of super-massive black hole binaries is expected to generate a
stochastic gravitational wave background (SGWB) in the pulsar timing array
(PTA) frequency range of $10^{-9}$--$10^{-7}$ Hz. Detection of this signal is a
current observational goal and so predictions of its characteristics are of
significant interest. In this work we use super-massive black hole binary
mergers from the MassiveBlackII simulation to estimate the characteristic
strain of the stochastic background. We examine both a gravitational wave
driven model of binary evolution and a model which also includes the effects of
stellar scattering and a circumbinary gas disk. Results are consistent with PTA
upper limits and similar to estimates in the literature. The characteristic
strain at a reference frequency of $1 yr^{-1}$ is found to be $A_{yr^{-1}} =
6.9 \times 10^{-16}$ and $A_{yr^{-1}} = 6.4 \times 10^{-16}$ in the
gravitational-wave driven and stellar scattering/gas disk cases, respectively.
Using the latter approach, our models show that the SGWB is mildly suppressed
compared to the purely gravitational wave driven model as frequency decreases
inside the PTA frequency band.
