Signatures of Circumbinary Disk Dynamics in Massive Black Hole Binary Populations

Abstract

Pulsar Timing Arrays (PTAs) have found compelling evidence for the existence of a Gravitational Wave Background (GWB): a ‘hum’ of low-frequency gravitational waves (GWs) that permeates the Universe. This signal is most likely emitted by inspiralling massive black hole binaries (MBHBs), the loudest GW sources in the Universe. For several decades, the dynamical processes governing the formation and evolution of MBHBs have been a topic of ongoing research, and a GWB detection poses urgent questions about their origin and fate: Do most MBHBs merge in a Hubble time? What are the expected properties of MBHB populations across cosmic time? And how is their origin and evolutionary history encoded in population statistics? This thesis takes a two-pronged approach to shed light on these questions.

Firstly, it is shown that the properties of MBHB populations and their multi-messenger observables are markedly altered by accretion from a circumbinary disk (CBD) during inspiral. Although the properties and abundances of the MBHB population depend on factors related to cosmological structure formation and galaxy morphology, accretion at parsec scales plays a crucial part in their orbital properties at merger. In particular, the interaction with a CBD is a driving factor in determining the mass ratios of the MBHBs that merge and produce GWs.

Secondly, new CBD-driven orbital evolution models are numerically derived using moving-mesh hydrodynamic simulations. This study results in a closed set of numerical equations that entirely govern the orbital evolution of binaries in CBDs. These new models cover an unprecedented parameter space, allowing for both mass ratio and eccentricity of the binary to evolve, and are applied to cosmologically inferred MBHB populations. It is shown that the CBD-driven evolution models presented in this thesis strongly impact the population characteristics of MBHBs and the GWB they produce. In particular, the discovery of a mass ratio dependent equilibrium eccentricity induced by CBD dynamics leaves a clear signature of CBD dynamics in MBHB populations.