The Discovery of Quasars

THE DISCOVERY OF QUASARS was a gradual process that took several years, from 1960 to 1963, and was eventually re solved through the discovery of their redshifts. I will first men tion the essential role that radio astronomy played in finding these ob jects and then describe the optical work that led to the discovery. Radio astronomy found its roots during World War II, when radar reflections had occasionally shown interference from unknown outside sources. After the war, radio astronomy observatories were set up at Jo drell Bank and Cambridge in the U.K. and in Australia. A number of discrete radio sources were soon discovered. The Revised 3C Catalogue of Radio Sources (3CR) provided positions and intensities for many sources accessible from the Northern Hemisphere. At Caltech's Owens Valley Radio Observatory, an interferometer of two 90-foot radio dishes was employed by Thomas Matthews and his collaborators in the late 1950s to produce more accurate positions of 3C sources. He would prepare optical observing charts in which the radio position was indi cated on a photographic image of the sky. An optical observer could then take a spectrum of one or more objects near the radio position and confirm the identification on the basis of the spectrum. Initially, all strong radio sources were identified with galaxies of the elliptical type, most of them with emission lines in their spectra. Galaxies are large stellar systems, like our own Milky Way Galaxy and the nearby Andromeda Galaxy. These building blocks of the Uni verse typically contain 100 billion stars, and much dark matter of un known nature. The total number of galaxies is likely of the order of 100 billion. The velocities of galaxies are all directed away from us, leading to a redshift in their spectra. As shown by Lemaitre in 1927 and confirmed by Hubble in 1929, the redshifts of galaxies are proportional to their distances, leading to the concept of the expanding Universe.