4.2. Energy Source
The question of the ultimate energy source for AGN stimulated creativity even before the discovery of QSO redshifts. The early concept of radio galaxies as galaxies in collision gave way to the recognition of galactic nuclei as the sites of concentrated, violent activity. Burbidge (1961) suggested that a chain reaction of supernovae (SN) could occur in a dense star cluster in a galactic nucleus. Shock waves from one SN would compress neighboring stars, triggering them to explode in turn. Cameron (1962) considered a coeval star cluster leading to a rapid succession of SN as the massive stars finished their short lives. Spitzer and Saslaw (1966), building on earlier suggestions, developed another model involving a dense star cluster. The cluster core would evolve to higher star densities through gravitational "evaporation", and this would lead to frequent stellar collisions and tidal encounters, liberating large amounts of gas. Additional ideas involving dense star clusters included pulsar swarms (Arons, Kulsrud, and Ostriker 1975) and starburst models (Terlevich and Melnick 1985).
Hoyle and Fowler (1963a, b) discussed the idea of a supermassive star (up to ~ 108 M) as a source of gravitational and thermonuclear energy. In additional to producing large amounts of energy per unit mass, all these models seemed capable of accelerating particles to relativistic energies and producing gas clouds ejected at speeds of ~ 5000 km s-1, suggestive of the broad emission-line wings of Seyfert galaxies. In this regard, Hoyle and Fowler (1963a) suggested that "a magnetic field could be wound toroidally between the central star and a surrounding disk." The field could store a large amount of energy, leading to powerful "explosions" and jets like that of M 87. Hoyle and Fowler (1963b) suggested that "only through the contraction of a mass of 107 - 108 M to the relativistic limit can the energies of the strongest sources be obtained."
Soon after, Salpeter (1964) and Zeldovich (1964) proposed the idea of QSO energy production from accretion onto a supermassive black hole. For material gradually spiraling to the innermost stable orbit of a nonrotating black hole at r = 6GM / c2, the energy released per unit mass would be 0.057 c2, enough to provide the energy of a luminous QSO from a reasonable mass. Salpeter imagined some kind of turbulent transport of angular momentum, allowing the matter to move closer to the hole, which would grow in mass during the accretion process.
The black hole model received limited attention until Lynden-Bell (1969) argued that dead quasars in the form of "collapsed bodies" (black holes) should be common in galactic nuclei, given the lifetime energy output of quasars and their prevalence at earlier times in the history of the universe. Quiescent ones might be detectable through their effect on the mass-to-light ratio of nearby galactic nuclei. Lynden-Bell explored the thermal radiation and fast particle emission to be expected in a disk of gas orbiting the hole, with energy dissipation related to magnetic and turbulent processes. For QSO luminosities, the disk would have a maximum effective temperature of ~ 105 K, possibly leading to photoionization and broad line emission. He remarked that "with different values of the [black hole mass and accretion rate] these disks are capable of providing an explanation for a large fraction of the incredible phenomena of high energy astrophysics, including galactic nuclei, Seyfert galaxies, quasars and cosmic rays."
Further evidence for relativistic conditions in AGN came from other theoretical arguments. Hoyle, Burbidge, and Sargent (1966) noted that relativistic electrons emitting optical and infrared synchrotron radiation would also Compton scatter ambient photons, boosting their energy by large factors. This would lead to "repeated stepping up of the energies of quanta", yielding a divergence that came to be known as the "inverse Compton catastrophe". This would be attended by rapid quenching of the energy of the electrons. They argued that this supported the idea of noncosmological redshifts. In response, Woltjer (1966) invoked a model with electrons streaming radially on field lines, which could greatly reduce Compton losses. He further noted that because "the relativistic electrons and the photons they emit both move nearly parallel to the line of sight, the time scale of variations in emission can be much shorter than the size of the region divided by the speed of light." The emission would also likely be anisotropic, reducing the energy requirements for individual objects.