Quasars and Other Active Nuclei

Abstract

Radio astronomy was responsible for the discovery of the first quasars. As long ago as 1960 several radio sources in the 3C catalogue had been noticed on account of their remarkably small angular size and were therefore particularly suitable for a search for an associated optical object. For 3C 48, optical exposures of the corresponding field indicated an object with a stellar appearance; its spectrum showed very strong emission lines that at first could not be identified. In 1962 Hazard, Mackey, and Shimmins, using the telescope at Parkes, succeeded in locating the source 3C 273 with great precision (better than 1″), thanks to a lunar occultation. Analysis of the light-curve profile at the beginning and end of the occultation showed, moreover, the existence of two components, A and B, separated by 20″; the second, 3C 273B, coincides exactly with a stellarlike object (m V ≈ 13) whose spectrum also turned out to have very strong emission lines. Schmidt discovered that these were in fact hydrogen lines that have been redshifted by an amount z = Δλ/λ0 = 0.158. Hence it was realized that if 3C 273 obeys the Hubble law, it is at an extremely large distance and has an enormous intrinsic luminosity (of the order of 1047 ergs s−1).