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 $10^{5}-10^{10}$ times the mass of the Sun. Accretion onto the supermassive black hole results in an active galactic nucleus (AGN), and accretion in some AGN drives highly-collimated jets of relativistic plasma. These jets extend over distances which can dwarf the galaxy hosting them, reaching up to megaparsec scales. Despite over five decades of study, open questions remain regarding jet formation and collimation, as well as particle acceleration in jets. In this dissertations I approach these issues at different scales, studying the dominant energy dissipation at small scales ($\sim 0.1-10$ pc), and studying the observable structure and dynamics of jets at large scales ($\gtrsim 10 $ pc). The first part of this 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. Using a simple yet robust diagnostic, I show that the location of energy dissipation in powerful jets is within the molecular torus. In the second part of this dissertations, I describe measurements of proper motions using optical imaging in an effort to understand the velocity distribution and structure of jets, which is ultimately necessary for a full understanding of their environmental impact. I focus on the study of three such jets, including that 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.