The most plausible mechanisms for energizing large, powerful extragalactic sources from AGNs involve well-collimated outflows near the symmetry axes of accretion disks around black holes. The black-hole potential well is needed (a) to generate the observed luminosity at high efficiency and (b) to provide the relativistic outflow velocities implied by super-luminal proper motions on parsec scales. The importance of well-focused outflows on large scales in these sources was only a promising theoretical idea (motivated by a few hints of jet-like structures in 3C sources) until VLA data showed that:
These results encourage models that ``unify" the known populations of compact and extended radio-loud AGNs assuming relativistic outflows, a range of orientations relative to the line of sight, and an obscuring torus whose geometry controls how close to the AGN we can probe with optical spectroscopy. It is now widely accepted that most, if not all, radio-loud AGNs are energized by jets that are initially relativistic.
It is much less clear whether, and under what conditions, outflows from AGN remain relativistic as they propagate away from the AGN and interact with the ISM, the IGM and/or their own ejecta. The enhanced VLA will allow an attack on this problem, as described below. The extent to which relativistic Doppler favoritism and aberration dominate the appearance of large (kpc-scale and greater) radio features in sources of different radio powers is also unclear. There is evidence that large-scale sources have some asymmetries that are relativistic illusions and others (size, spectral asymmetries) that are intrinsic. To understand the dynamics and the evolution of large-scale radio sources in real circumgalactic environments, we must determine how jets interact with their surroundings in sources with a wide range of powers and in a wide range of environments.