This review has given a brief summary of the properties of galaxies as
the radio background. In conclusion, it is worth emphasizing three points:
- Although some factors such as galaxy mass and Hubble type strongly
galaxy to host a radio-loud AGN, we still have no definite understanding of why
this should be so. Other `distinguishing marks' of radio galaxies might
be helpful in this process, but few if any are clearly established.
- With certain exceptions (such as the situation at z
2), we have a good statistical
description of how the abundance of radio AGN evolves. Again, though, we are
very far from understanding why active nuclei found it so much easier to
function at high redshift.
- High-redshift radio galaxies should probably not be thought of as
in any way
primeval. If we ignore the few dozen most luminous sources in the
the optical/IR properties of high-redshift radio galaxies are consistent
of radio-quiet ellipticals. They appear to be red and old: theories in
massive galaxies complete their star formation at z
4 are required.