## Theses and Dissertations

August 2017

Dissertation

#### Degree Name

Doctor of Philosophy

Physics

Xavier Siemens

#### Committee Members

Phil Chang, Jolien Creighton, Dawn Erb, David Kaplan

#### Keywords

Binary Supermassive Black Holes, Gravitational Waves, Pulsar Timing Arrays

#### Abstract

The recent direct detections of gravitational waves (GWs) from merging black holes by the Laser Interferometer Gravitational-wave Observatory (LIGO) marks the beginning of the era of GW astronomy and promises to transform fundamental physics. In the coming years, there is hope for detections across the mass scale of binary black holes.

Pulsar Timing Arrays (PTAs) are galactic-scale low-frequency (nHz - $\mu$Hz) GW observatories, which aim to directly detect GWs from binary supermassive black holes (SMBHs) ($\gtrsim 10^{7} \msun$). The frequency and black hole mass range that PTAs are sensitive to is orders of magnitude different from those LIGO is observing, making PTAs a comparable observatory on the GW spectrum. Understanding the link between binary SMBHs and the gravitational radiation detected by PTAs is crucial to the community's capability of making meaningful scientific statements using PTA observations. This dissertation discusses the creation of a state-of-the-art observational-based simulation framework built to provide critical answers to many open questions surrounding the link between PTA data and binary SMBHs.

Binary SMBHs are predicted products of galaxy mergers, and are a crucial step in galaxy formation theories. Recent PTA upper limits on the gravitational radiation in the nanohertz frequency band are impacting our understanding of the binary SMBH population. But as upper limits grow more constraining, what can be implied about galaxy evolution? In this dissertation, I will provide insights into this question by investigating which astrophysical parameters have the largest impact on GW predictions, developing direct translations between PTA limits and measured values for the parameters of galaxy evolution, and exploring how the use of different galaxy evolution parameters effects the characterization of the GW signal.

During the extended interaction between SMBHs and their host galaxy throughout inspiral, there is the potential for many electromagnetic tracers to accompany the binary's evolution. This dissertation also incorporates models of electromagnetic radiation from binary SMBHs to investigate the potential for jointly detecting a binary's electromagnetic and gravitational radiation. The detection of a single `multi-messenger' source would provide a unique window into a pivotal stage of galaxy evolution, and would revolutionize the understanding of late-stage galaxy evolution.

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