Thesis Dissertation Defense

Motivated by observational searches for potential gravitational-wave (GW) signals from massive black binary (MBHB) coalescences, we developed a model to describe orbital evolution of MBHBs. In this thesis, we use the model developed to determine how the properties of the merger remnant galaxy and the orbital configuration of MBHs affect the likelihood for and timescale to a coalescence. By varying galactic properties and orbital configurations of MBHBs in the model, we built a parameter space that contains 40,000 model galaxies, spans a wide range in initial orbital eccentricities and includes both prograde and retrograde orbits. We used these models to acquire a comprehensive view of how different types of orbital decay mechanisms impact the MBHB evolution. We estimated the LISA detection rates for different binary orbital configurations in the absence and presence of radiation feedback and explored the properties of MBHBs that are most likely to be detected as GW sources by applying the model on MBH pairs from the IllustrisTNG simulation. Finally, we use the model to quantify the electromagnetic (EM) detectability of dual active galactic nuclei (dAGNs). By tracking how the EM detectability varies with galaxy and orbital properties, we provided a convenient way to select dAGN candidates that evolve into GW sources. These kinds of predictions will be crucial for the future and present EM and GW observatories, for they will indicate where to look for possible MBH coalescences or the most detectable dAGNs.