Published in "Extragalactic Background Radiation", Space Telescope Science Institute Symposium Series 7, 1995, eds. D. Calzetti, M. Livio and P. Madau
Abstract. This paper is in two unequal halves. After dealing with the possibility of a genuine continuum background at 1 cm, and showing that it is unlikely to arise in interesting circumstances, the remainder of the discussion is devoted to discrete radio sources, and their consequences for cosmology. Three main issues are considered: (i) what makes a galaxy radio loud?; (ii) what do we know about how the population of radio-loud galaxies has changed with epoch?; and (iii) what can observations of high-redshift radio galaxies tell us about general questions of galaxy formation and evolution? The main conclusion is that radio galaxies are remarkably ordinary massive ellipticals. The high-redshift examples are generally old and red and do not make good candidates for primeval galaxies.
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