Investigating the Dynamics of Extragalactic Jets from Parsec to Kiloparsec Scales: Novel Constraints on Jet Acceleration and Emission

Abstract

Actively accreting super-massive black holes, known as active galactic nuclei or AGN, are known to produce bipolar jets of relativistic plasma which carry matter and energy out of the central nucleus of a galaxy to scales far beyond the host to distances of a megaparsec or more, close to 10 orders of magnitude in gravitational radius. However, in spite of tremendous progress over the last several decades in astronomical observations of jets, it is relatively unknown how these jets form, accelerate and maintain strong collimation through the kiloparsec scale. Studies of motions of bright compact inhomogeneities at different positions in these jets (``proper motions") can, in principle, be used to produce model-independent constraints on the Lorentz factor ($\Gamma$) of the bulk motion at different spatial scales in the jet. This inferred velocity profile of the jet, in addition to constraining theories of jet acceleration-deceleration, is vital in understanding the origin of high-energy emission (X-rays and beyond) from extragalactic jets. With a view to leveraging the rich decades-spanning archive of the Very Large Array (VLA) telescope in particular, this thesis develops new radio interferometric techniques to measure jet velocities accurately beyond the few hundred parsec scale. I have initiated a catalogue of such measurements known as CAgNVAS: Catalogue of proper motions in AGN jets with Very large Array Studies. In this thesis I present in detail the velocity profiles of two extragalactic jets along with theoretical models of the observations. In both the jets, I find evidence of acceleration in a kpc-scale matter-dominated jet for the first time, with insights into the nature of the multi-wavelength spectrum of kpc-scale jets. The techniques developed and the science produced from this thesis will enable a much richer study of large-scale jets as seen by moderate-baseline interferometers like the Very Large Array.