Supermassive Black Hole Binaries: Environment and Galaxy Host Properties
  of PTA and eLISA sources
  Supermassive black hole (BH) binaries would comprise the strongest sources of
gravitational waves (GW) once they reach <<1 pc separations, for both pulsar
timing arrays (PTAs) and space based (SB) detectors. While BH binaries
coalescences constitute a natural outcome of the cosmological standard model
and galaxy mergers, their dynamical evolution is still poorly understood and
therefore their abundances at different stages. We use a dynamical model for
the decay of BH binaries coupled with a cosmological simulation and
semi-empirical approaches to the occupation of haloes by galaxies and BHs, in
order to follow the evolution of the properties distribution of galaxies
hosting BH binaries candidates to decay due to GWs emission. Our models allow
us to relax simplifying hypothesis about the binaries occupation in galaxies
and their mass, as well as redshift evolution. Following previously proposed
electromagnetic (EM) signatures of binaries in the subpc regime, that include
spectral features and variability, we model possible distributions of such
signatures and also set upper limits to their lifespan. We found a bimodal
distribution of hosts properties, corresponding to BH binaries suitable to be
detected by PTA and the ones detectable only from space missions, as eLISA.
Although it has been discussed that the peak of eLISA sources may happen at
high z, we show that there must be a population of such sources in the nearby
Universe that might show detectable EM signatures, representing an important
laboratory for multimessenger astrophysics. We found a weak dependence of
galaxy host properties on the binaries occupation, that can be traced back to
the BH origin. The combination of the host correlations reported here with the
expected EM signal, may be helpful to verify the presence of nearby GW
candidates, and to distinguish them from 'regular' intrinsic AGN variability.
