Surveys of extragalactic radio sources

Nearly all discrete radio sources are extragalactic and lie at cosmological distances. Most sources stronger than ~1 mJy at 1.4 GHz are powered by active galactic nuclei (AGNs), while the majority of fainter sources are in starburst galaxies. Recent all-sky surveys have detected most of this AGN population, and smaller surveys are now sensitive enough to probe the evolution of star formation in selected areas. Sensitive new radio surveys will complement far-infrared, ultraviolet, and X-ray surveys now being made in “cosmic windows” having exptionally low interstellar extinction. They will help define the evolution of star-formation and dust-shrouded AGNs over cosmological timescales