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PhD Defense: Adam Leah Harvey

Monday, June 21, 2021
10:00 AM – 1:00 PM
Off Campus : via WebEx
ADVISORS: Dr. Eileen T. Meyer and Dr. Markos Georganopoulos

TITLE: Dynamics and Energy Dissipation in Extragalactic Jets

ABSTRACT: We now know that in the center of almost all galaxies lies a supermassive black hole with a mass of 10^5 to 10^10 times the solar mass. Accretion onto the supermassive black hole in some active galactic nuclei (AGN) drives highly-collimated jets of relativistic plasma. These jets extend over distances which can dwarf the galaxy hosting them, reaching the kiloparsec, and in some cases megaparsec scale. There are open questions regarding the physics of these jets at all length scales. Particularly challenging have been questions of jet formation, collimation, and particle acceleration. In this dissertation I approach these issues at different scales, studying the dominant energy dissipation at small-scales (~0.1-10 pc), and studying the observable structure and dynamics of the jet at large-scales (>~10 pc).

The first part of my work is on localizing the site where powerful jets dissipate a significant fraction of their kinetic energy into gamma-ray radiation. The location of energy dissipation in powerful extragalactic jets has long been a matter of debate, with implications for the theory of the structure and formation of jets. Previous studies have been unable to constrain the location between possibilities ranging from within the sub-parsec-scale, where the dominant photon field illuminating the jet comes from the so called broad-line region, to the parsec-scale, where the dominant photon field comes from the thermally radiating molecular torus just beyond it. I show using a simple yet robust diagnostic that a location of energy dissipation within the molecular torus, beyond the broad-line region, is required in powerful jets.

In the second part of this dissertation, I describe measurements of proper motions using optical imaging in an effort to understand the velocity distribution, structure, and environmental impact of jets. I focus on the study of three such jets, including the jet of M87 (the earliest discovered jet) where I find evidence for recurring stationary pressure-gradient-driven shocks as well as a helical pattern in the plasma flow. These findings place constraints on particle acceleration in jets, showing that multiple sites of acceleration exist along the jet of M87.

Defense will be held using WebEx.