Supermassive Black Holes in Active Galactic Nuclei

Abstract

Quasars are among the most energetic objects in the Universe. We now know that they live at the centers of galaxies and that they are the most dramatic manifestation of the more general phenomenon of active galactic nuclei (AGNs). These include a wide variety of exotica such as Seyfert galaxies, radio galaxies, and BL Lacertae objects. Since the discovery of quasars in 1963, much effort has gone into understanding their energy source. The suite of proposed ideas has ranged from the relatively prosaic, such as bursts of star formation that make multiple supernova explosions, to the decidedly more colorful, such as supermassive stars, giant pulsars or "spinars," and supermassive black holes (hereinafter BHs). Over time, BHs have gained the widest acceptance. The key observations that led to this consensus are as follows. Quasars have prodigious luminosities. Not uncommonly, L ∼ 10 46 erg s −1 ; this is 10 times the luminosity of the brightest galaxies. Yet they are tiny, because they vary on timescales of hours. From the beginning, the need for an extremely compact and efficient engine could hardly have been more apparent. Gravity was implicated, because collapse to a black hole is the most efficient energy source known. The most cogent argument is due to Donald Lynden-Bell (1969, Nature, 223, 690). He showed that any attempt to power quasars by nuclear reactions alone is implausible. First, a lower limit to the total energy output of a quasar is the energy, ∼ 10 61 erg, that is stored in its radio-emitting plasma halo. This energy weighs 10 40 g or 10 7 M ⊙. But nuclear reactions produce energy with an efficiency of only ǫ = 0.7 %. Then the waste mass left behind in powering quasars would be at least M • ≃ 10 9 M ⊙. Lynden-Bell argued further that quasar engines are 2R ∼ 10 62 erg. As Lynden-Bell noted, "Evidently although our aim was to produce a model based on nuclear fuel, we have ended up with a model which has produced more than enough energy by gravitational contraction. The nuclear fuel has ended as an irrelevance." We now know that the total energy output is larger than the energy that is stored in a quasar's radio source; this strengthens the argument. Meanwhile, a caveat has appeared: the objects that vary most rapidly are now thought to contain relativistic jets that are beamed at us. This boosts the power of a possibly small part of the quasar engine and weakens the argument that the object cannot vary on timescales less than the light travel time across it. But this phenomenon would not occur at all if relativistic motions were not involved, so BH-like potential wells are still implicated. These considerations suggest that quasar power derives from gravity.