Possibly the most exciting areas for research with the enhanced VLA are the evolution of galaxies with redshift and related questions about the epoch of galaxy formation and the detection of protogalaxies.
For many years we have known that the radio source population evolves with cosmological epoch. In particular, the density of sources increases rapidly with redshift. This seems to be true for other types of AGNs as well such as optically selected, often radio weak or silent, QSOs.
Optically, we find that only a little below the 3CR flux limit, most sources are at high redshifts, and often as high as z = 3 to 5. Before 1988 we had little information on galaxies in the early universe. Since then there has been an explosion in the number of stellar systems known at high redshift, with almost 100 galaxies now identified at 2, and about 10 at 3, corresponding to an epoch when the universe was only about 10 its present age. Most of these galaxies are identified with powerful radio sources although some high redshift galaxies have been identified via their infra-red emission and others have been found associated with quasar absorption line systems.
Many of these systems display characteristics expected for young galaxies ( yrs) undergoing a massive starburst, including: halos of ionized gas up to 100 kpc in extent, dust masses in excess of solar masses , and flat optical spectral energy distributions indicative of a young stellar population. Star formation rates in excess of solar masses have been inferred for some systems.
As mentioned earlier, the HST has been imaging clusters of galaxies beyond z = 0.4 and finding evidence confirming the Butcher-Oemler effect, the blueing of cluster galaxy populations with redshift, is related to increased star formation in clusters with redshift. The HST data also show that galaxies themselves have different shapes as we look further back.
Direct observational study of the evolution, and possibly of the formation, of galaxies is clearly becoming possible. The enhanced VLA could play a very important rôle in this exiting arena.
We discussed one key investigation already-a deep H I survey to examine the evolution of gas content of galaxies out to z. Four other areas of study could be transformed by the enhanced VLA: