Rotating Supermassive Black Holes (2)

Large Central Masses

One thing that makes us rather certain that massive black holes can exist in the center of active galaxies is that there is strong observational evidence for enormous amounts of unseen mass in the centers of many galaxies (whether active or not). One finds that the centers of galaxies often contain billions of solar masses in very compact regions. Since there is far less luminosity in such regions than would be expected if this mass were in the usual distribution of gas, dust, and stars, a supermassive black hole is considered by most astronomers to be the simplest explanation that fits the observations (see the box below). As noted in the adjacent box, even supermassive black holes are so tiny that we could not resolve them with present instruments (even if they were not obscured by gas, dust, and radiation). Thus, we must study them using indirect evidence.

The Standard AGN Model

The black hole itself occupies a tiny region in the center (its size is exaggerated for clarity in this figure). It is surrounded by a dense torus of matter whirling around the hole and a flattened accretion disk inside of that where the matter whirls even faster before disappearing into the black hole. On the polar axes of the rotating black hole there are jets of matter being ejected as part of the matter in the accretion disk is sucked into the black hole and part is flung out at high velocity in the jets.

Why Rotating Black Holes? We have asserted that the most plausible candidate for powering an AGN is a rotating, supermassive black hole. But an unwritten rule of science is to "question authority". That is, you should not accept that AGNs are powered by black holes just because your astronomy book says so! You should ask what the evidence is to support this hypothesis. We shall present specific circumstantial but very strong evidence in the later sections of this module, but here let us address two aspects of this question at a general level. First, why a supermassive black hole? Second, why does it need to be rotating? Why a Black Hole? Why a black hole? In essence, because it is the only plausible way that we know to produce the compact energy source that data require, and still be consistent with all observations. The primary reason for this is that a (rotating) black hole is one of the most efficient ways known to convert mass to energy. Einstein's famous equation ensures us that if mass is converted to energy, the amount of energy we get is given by E = mc2. However, to utilize this, we must have a physical mechanism that converts mass to energy. (For example, the mass-energy relation was published by Einstein in 1905, but it was the late 1930s before the first physical process--nuclear fission--was discovered that could actually convert mass to energy in significant quantities.) Mass Conversion Efficiencies Physical Process Efficiency Hydrogen Fusion 0.007 Black Hole Accretion ~ 0.10 Matter-Antimatter Annihilation 1.0 In most processes that convert mass to energy, only a small part of the available mass is converted. The fraction that is converted is termed the efficiency of the conversion. The left-hand table gives some efficiencies for mass-energy conversion in several physical processes. Notice that hydrogen fusion is highly inefficient. In the fusion of hydrogen to helium, only 0.007 (less than 1 percent) of the available mass is converted to energy. The rest remains as mass. On the other hand, calculations indicate that 10 percent of the mass falling into a black hole can be radiated as energy (in some cases it is even more, but let's take this as an average figure). That is more than 10 times as efficient as hydrogen fusion. The only process that we know to be more efficient is matter-antimatter annihilation, which converts 100 percent of the mass to energy. But AGNs can't be powered by annihilation because there are signatures in the gamma-ray spectrum that would tell us that matter and antimatter were annihilating to produce the AGN's energy and these are not seen. There have been some attempts to explain the power source of AGNs as a large number of supernova explosions in the centers of galaxies. That might just barely produce enough energy, but the details of AGN observations do not support this idea very well. Most astronomers conclude that only a black hole of very large mass can produce the required energy in a manner consistent with the properties of AGNs. Because of the high efficiency of a black hole engine, calculations indicate that even luminous AGNs can be powered by an accretion rate of only about two solar masses a year. But Why Rotating? Even if we accept the argument from above that the AGN energy source is a supermassive black hole, why does it need to be rotating? There are at least three arguments favoring a rotating black hole. First, the efficiency for extracting energy from a black hole by dropping mass into it can be much higher for a rotating black hole (Kerr black hole) than for a nonrotating one (Schwarzschild black hole). Loosely, matter dropped onto a nonrotating black hole gains large energy as it accelerates in the gravitational field, but it falls through the event horizon before it can radiate much of this energy. On the other hand, for a rotating black hole it is possible for matter to swirl around in the accretion disk and radiate significant amounts of energy before part of the matter is sucked through the event horizon and part is ejected in jets (which carry additional energy extracted from the rotating black hole). Second, the most plausible mechanisms for focusing the tightly collimated jets seen emerging from many AGNs, and explaining how they can point in the same direction for millions of years, requires the strong magnetic fields and gyroscope effect that a rapidly rotating central engine could produce (Recall that a gyroscope resists any attempt to change the direction of its rotation axis. That is one reason why bicycles are easy to ride: the wheels act as gyroscopes). Third, because the formation of black holes generally involves the collapse of matter to regions of small diameter, it would be difficult for a black hole to avoid having a high spin rate. Conservation of angular momentum requires the collapsing matter to spin more rapidly if it had any initial angular momentum (recall the spinning ice skater analogy that we have invoked numerous times now to illustrate angular momentum conservation). Therefore, most astronomers believe that rotating, supermassive black holes power active galactic nuclei and quasars. In the remainder of this module, we shall present more detailed evidence to support this view.