Astronomy 218 Quasars AGN History The first detection of an emission spectrum from a galaxy was 1908. By the 1920s, several more had been added. Seyfert introduced the classification of active galactic nucleus in 1943, showing that ~1% of spirals had AGNs. The development of radio astronomy after World War II led to the discovery of radio galaxies, as these were among the brightest radio sources in the sky. Quasi-Stellar Searches for the optical counterparts to strong radio sources revealed many radio galaxies. Improvements in radio telescopes through the 1950s revealed many more, fainter radio galaxies. In 1960, Matthews & Sandage found a 16th-magnitude point-like optical counterpart to 3C 48. This was joined in 1963 by a similar companion to 3C 273. They were dubbed Quasi-Stellar 1” Radio Sources (QSRs). Peculiar Spectrum The optical spectra of QSRs (pronounced quasars) were perplexing, exhibiting strong emission lines that corresponded to no known elements. Later in 1963, Maarten Schmidt (1929− ) identified the Balmer series, but at a redshift of z = 0.158, indicating a speed 14.6% of the speed of light. It was soon realized that quasar spectra were those of active galactic nuclei, but enormously redshifted. Greenstein & Matthews revealed 3C 48 had z = 0.367. Extremely Luminous Solving the spectral problem introduced a new problem. Applying Hubble’s law to a redshift of z = 0.158 yields a distance of 620 Mpc for 3C273! 3C 48’s redshift of z = 0.367 is equivalent to a Hubble distance of 1.3 Gpc. At the time of this discovery, 3C 48 was among the most distant objects yet discovered. Quasars must be among the most luminous objects (L ~ 1038‒1041 W) in the universe, though they appear quite faint at visible wavelengths because of their distance. Quasar Spectra With the redshift revealed, the optical spectra of nearby quasars look like broad line Active Galactic Nuclei, although higher redshifts move Lyman and other UV lines into the visual wavelength range. Overall, the spectral energy distributions (SEDs) of quasars are similar to those of radio-loud AGN, though brighter and more variable. For example, this SED shows the average and range of fluctuations for 3C273. Finding Galaxies While some early QSR optical counterparts showed “fuzziness” suggesting nebulosity, not until 1982 (by Boronson & Oke for 3C 48) was it firmly established that the nebulosity was stellar and at a similar redshift. This cemented the picture of quasars as distant AGN. The quasars in these images are shown with their host galaxies. Quasi-Stellar Object Like AGN in general, the radio-loud QSRs are joined by a larger class of similar, but radio-quiet objects. For quasars, these historically were termed Quasi-Stellar objects (QSOs), to differentiate them from QSRs, though QSO and quasar are now used almost interchangeably by many astronomers. In modern day, Radio-Quiet Quasars (RQQ), which make up > 90% of quasars, are distinguished from Radio-Loud Quasars (RLQ) to avoid the confusion between QSO and quasar. Superluminal Expansion With the development of Very-Long-Baseline Interferometry (VLBI) in the 1970s, it became possible to resolve the jets of quasars. For 3C273, this showed knots with proper motion of μ ~ 0.7 mas yr−1. At a distance d > 600 Mpc, this implies a tangential velocity, −1 �t ≈ 4.74 km s μ” (d/pc) ≈ 4.74 (6 × 108 pc)(7 × 10-4” yr−1) ≈ 2 × 106 km s−1 ≈ 7 c Such superluminal velocities restarted the simmering debate over the true distance to quasars, but ultimately the solution is relativistic geometry. Trick of the Light A Photons emitted at t1 from B1 arrive at A at time Photons emitted at t2 from B2 arrive at A at time The time interval between the two observations is Δϕ d θ