27.2 Supermassive black holes: what quasars really are

Learning objectives

By the end of this section, you will be able to:

• Describe the characteristics common to all quasars
• Justify the claim that supermassive black holes are the source of the energy emitted by quasars (and AGNs)
• Explain how a quasar’s energy is produced

In order to find a common model for quasar    s (and their cousins, the AGNs), let’s first list the common characteristics we have been describing—and add some new ones:

• Quasars are hugely powerful, emitting more power in radiated light than all the stars in our Galaxy combined.
• Quasars are tiny, about the size of our solar system (to astronomers, that is really small!).
• Some quasars are observed to be shooting out pairs of straight jets at close to the speed of light, in a tight beam, to distances far beyond the galaxies they live in. These jets are themselves powerful sources of radio and gamma-ray radiation.
• Because quasars put out so much power from such a small region, they can’t be powered by nuclear fusion the way stars are; they must use some process that is far more efficient.
• As we shall see later in this chapter, quasars were much more common when the universe was young than they are today. That means they must have been able to form in the first billion years or so after the universe began to expand.

The readers of this text are in a much better position than the astronomers who discovered quasars in the 1960s to guess what powers the quasars. That’s because the key idea in solving the puzzle came from observations of the black holes. The discovery of the first stellar mass black hole in the binary system Cygnus X-1 was announced in 1971, several years after the discovery of quasars. Proof that there is a black hole at the center of our own Galaxy came even later. Back when astronomers first began trying to figure out what powered quasars, black holes were simply one of the more exotic predictions of the general theory of relativity that still waited to be connected to the real world.

It was only as proof of the existence of black holes accumulated over several decades that it became clearer that only supermassive black hole    s could account for all the observed properties of quasars and AGNs. As we saw in The Milky Way Galaxy , our own Galaxy has a black hole in its center, and the energy is emitted from a small central region. While our black hole doesn’t have the mass or energy of the quasar black holes, the mechanism that powers them is similar. The evidence now shows that most—and probably all—elliptical galaxies and all spirals with nuclear bulges have black holes at their centers. The amount of energy emitted by material near the black hole depends on two things: the mass of the black hole and the amount of matter that is falling into it.

If a black hole with a billion Suns’ worth of mass inside (10 ⁹ M _Sun ) accretes (gathers) even a relatively modest amount of additional material—say, about 10 M _Sun per year—then (as we shall see) it can, in the process, produce as much energy as a thousand normal galaxies. This is enough to account for the total energy of a quasar. If the mass of the black hole is smaller than a billion solar masses or the accretion rate is low, then the amount of energy emitted can be much smaller, as it is in the case of the Milky Way.