Discovery and Taxonomy of AGNs The major classes of AGN (radio galaxies, quasars, Seyfert nuclei, and BL Lac objects) were found in rather different ways. Radio galaxies were found as the optical counterparts of radio sources from early surveys - see Matthews, Morgan, and Schmidt 1964 (ApJ 140, 35). Their optical spectra may or may not show emission lines; when seen, the lines may be narrow- or broad-lined, giving continuity to QSOs and Seyferts of types 1 and 2. The radio structures are often twin-lobed, showing one or two jets tracing back to the nucleus when observed with high resolution and dynamic range. Quasars (quasi-stellar radio sources) were first thought to be radio-loud stars, when interferometry or lunar-occultation positions allowed precise identification (as for 3C 273, Hazard, Mackey and Shimmins 1963 Nature 197, 1037). In this case, the optical identification was a 13th-magnitude stellar object with optical jet , approximately matching the radio structure. The spectrum was finally recognized by Schmidt (1963 Nature 197, 1040) and Greenstein and
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