Extragalactic radio astronomy spans the study of normal stellar phenomena in external galaxies, of the intense, extended ejecta of radio-loud AGNs, and of the gaseous environments of galaxies and clusters of galaxies. Our ability to trace the more radio-luminous phenomena to high redshifts interweaves the study of the evolution of radio sources and their environments with the study of the large-scale structure and evolution of the universe itself. Extragalactic astrophysics and cosmology are thus both mutually entangled and mutually illuminating.
We are now entering an age in extragalactic radio astronomy in which we can detect the normal stellar phenomena (supernovae and their remnants, synchrotron and thermal emissions from disks) in many external galaxies. The gas content of normal galaxies can be studied to redshifts high enough to observe some effects of cosmological evolution directly. The radio-loud populations, and examples of dusty-star-forming galaxies, can be examined out to redshifts at which the universe was only 10% of its present age. This allows direct and dramatic probes of how the observable properties of starbursts, radio jets and lobes depend on cosmological epoch.
We have also uncovered ways to use extragalactic radio sources to probe the environments in groups and clusters of galaxies, through their direct bending and distortions by intergalactic gas, through their Faraday rotations, and through gravitational lensing of distant systems by foreground matter.
The enhanced VLA can make enormous contributions to all these aspects of extragalactic research. In some arenas, such as the study of large-scale jets and lobes formed by radio-loud AGN's, it will allow us to advance to the next level of detail in astrophysical modeling of individual objects-its improved resolution and sensitivity will allow new and critical tests of more realistic models. In others, such as the study of supernovae and their remnants in external galaxies, the sensitivity of the enhanced VLA will give access for the first time to a representative population of the objects. Perhaps most exciting of all are the opportunities that the improved sensitivity, expanded correlator capacity and frequency coverage will give us to observe the effects of cosmological evolution directly in a variety of contexts, ranging from the gaseous content of normal galaxies and galaxy clusters to starbursts and the large-scale sources associated with radio-loud AGN's.
The VLA was originally designed with the needs of extragalactic astronomy very much in mind in terms of defining resolution and sensitivity targets. Its spectacular success in this field of research has by no means ``wrapped up'' its vital rôle in the study of the distant universe. Rather, it has allowed us to glimpse the even more exciting possibilities that exist for extragalactic science with the more capable instrument that is now being proposed. This Chapter reviews these possibilities.