4 - Quasars and AGN

Summary

Some celestial objects, later recognized as quasars, were catalogued back in 1887, and their extragalactic nature was discussed as early as 1960. However, the large measured redshift of 3C 48 was rejected, largely because it implied an unrealistically high radio and optical luminosity. Instead it was assumed to be a relatively nearby, less luminous galactic radio star. Following the 1962 observations of lunar occultations of the strong radio source 3C 273 at the Parkes radio telescope and the subsequent identification with an apparent stellar object, Martin Schmidt recognized that 3C 273 had an unmistakable redshift of 0.16. Due to an error in the calculation of the radio position, the occultation position actually played no direct role in the identification of 3C 273, although it was the existence of a claimed accurate occultation position that motivated Schmidt’s 200 inch telescope investigation and his determination of the redshift. Later radio and optical measurements quickly led to the identification of other quasars with increasingly large redshifts, although the nature of the quasar redshifts remained controversial for decades.