8. UNIFIED MODEL

The energy source: Einstein's famous formula, E = mc², gives the energy that can be produced as radiation if one can convert mass to energy. Stars do this by burning hydrogen into helium, helium into carbon and oxygen, and ultimately into iron. But the fraction of the original hydrogen mass that can be converted into energy by such fusion reactions is only about 1%. The other 99% of the mass is locked up as iron, the "ash" of the reactions, which has no more energy to release by fusion.

The ultimate source of energy in the universe is gravity. As matter swirls into a black hole, it forms an "accretion disk," a vortex of gas that we have already encountered when we discussed stellar black holes in Lesson 7. As the gas swirls through this disk into the black hole, it becomes extremely hot and radiates much of this heat energy as X-rays and gamma rays. According to theoretical calculations, the gas can release up to 30% of its rest-mass energy as radiation as it winds its way through the disk into a rapidly rotating black hole.

That's a lot of power -- about 30 times as much as the same amount of matter could produce by fusion reactions. This result implies that one solar mass per year of gas flowing through such a disk would be sufficient to produce about 3 x 10¹² solar luminosities -- roughly 15 times the luminosity of the entire Milky Way galaxy!

Most of the luminosity is generated by gas flowing in the accretion disk within a few times the Schwarzschild radius of the black hole. The Schwarzschild radius of a 10⁸ solar mass black hole is 2 Astronomical Units (see Lesson 7).

Feeding the monster: But what is the source of this mass flow? We have already seen several examples of relatively quiet galaxies with disks of gas and dust around supermassive black holes. But the disks we see with the HST are orbiting at distances of tens of light years, hundreds of thousands of times greater than the Schwarzschild radius. Evidently, most of the gas in these disks is simply orbiting the black hole, not spiraling in.

There are two obvious mechanisms to provide a supply of gas very close to a supermassive black hole. One is stellar collisions. If the density of stars near the black hole is great enough, the stars may collide with each other. (Red giant stars will collide more often than main sequence stars because they are so much bigger.) The outer envelopes of the stars will be torn off during such collisions and will provide a source of gas for the inner disk. Another mechanism is collisions of galaxies. Then, the gravitational and pressure forces from passing stars of the second galaxy may disturb a pre-existing gas disk orbiting a supermassive black hole and cause it gas to spiral in. Alternatively, pressure forces due to colliding interstellar gas clouds may cause gas to flow inwards. There is some evidence that collisions of galaxies play a role in the AGN phenomenon. As we have already described, the galaxies around DRAGNs and quasars often appear to be highly disturbed, perhaps colliding galaxies. That is consistent with the fact that quasars appear to be far more common in the universe when it was about 20% of its present age. We know that galaxies were colliding much more often then than they are today. At any rate, we are sure that there are many more quiet galaxies containing supermassive black holes than active galaxies. It follows that many galaxies that were once quasars, radio galaxies, or Seyfert galaxies have settled down into normal elliptical and spiral galaxies. They are quiet now because the monster has no supply of mass. But he is lurking there all the same, ready to start breathing fire again if he is disturbed by a passing galaxy.

Two move animations of a quasar central engine. The first one stars with HST pictures of Centarus A source, and switches to an animation of a black hole surrounded by an accretion disk. The second one animates a birth of quasar in a spiral galaxy. Note: both animations require QuickTime 5.0, and so may not run on every computer.

The Eddington Limit: There is a limit to the luminosity of an active galactic nucleus. This limit was originally derived by Sir Arthur Eddington in another context. Eddington realized that if a star became luminous enough, the radiation leaking out from the center of the star would exert an outward force on the star's envelope that would exceed the inward force due to gravity. Then the star would simply fly apart. Eddington calculated that there could be no star more massive than about 100 Suns as a result of this outward force, and that seems to be the case.

The same logic applies to the radiation from gas falling into a black hole. If the luminosity becomes great enough, it will push the gas away from the black hole and disrupt the accretion disk. This limiting luminosity, which we call the Eddington Limit, is proportional to the mass of the black hole and is about 3 x 10¹² solar luminosities for a black hole of mass 10⁸ solar masses. It appears that the many active galaxies are radiating with luminosities close to the Eddington limit. It also follows that if we see an active galaxy with very high luminosity, it must contain a very massive black hole.

Jets: As we have seen, many active galaxies, including M87, DRAGNs, Seyfert galaxies, and 3C273, have narrow jets of electrons that move close to the speed of light and extend far into intergalactic space. We do not have a very good understanding of the mechanism that causes these jet-like outflows, but we can see that the jets are perpendicular to the accretion disks and that they are formed very near the center of the disks. We suspect that magnetic forces play an important role in confining the jet outflows to the polar directions and in accelerating the electrons to relativistic speeds.

Most DRAGNs have two such jets streaming in opposite directions. But in some objects, such as 3C273 and M87, we see only one jet. We believe this is an illusion, and that these objects probably also have opposing twin jets. But Einstein's theory of relativity tells us that a jet will appear much brighter if it is moving toward us at nearly the speed of light. So we see only the jet that is moving toward us, and the opposite one is too faint to see. In fact, blazars are probably active galactic nuclei in which one jet is pointing nearly exactly in our direction. In that case, the jet not only appears much brighter than it would viewed from another direction, but its radiation comes out at higher energy, and its brightness appears to vary on a much shorter timescale. All these characteristics are natural consequences of Einstein's theory of special relativity. The elliptical galaxy M87, which is not an exceptionally luminous AGN, might be seen as a blazar by astronomers living on some planet directly downstream from its jet.

Spectra: AGNs are luminous because of the energy released by gas as it flows into a supermassive black hole. But what accounts for the variety of different spectra that we see in AGNs? We believe that much of this variety can be explained by the orientation of the accretion disk.

X-ray emission: The gas in the inner disk near the Schwarzschild radius is very hot and radiates mostly X-rays. Typically, the spectrum X-rays from such hot gas has a strong emission line at energy 6.4 keV due to iron atoms in the gas. In 1995, Japanese astronomers using the ASCA X-ray satellite observed the X-ray spectrum of a type I Seyfert galaxy called MCG-6-30-15. They found a very broad emission line with a peak at 6.4 keV and a broad redshifted wing extending down to about 4 keV.

The X-ray line profile is shown in the above figure (courtesy of Chris Reynolds). The top image is an optical picture of MCG-6-30-15. The middle image is a theoretical model for the inner part of the accretion disk that is producing the X-ray emission line. A black hole is at the center. The X-ray emission is much brighter on the left because the disk is rotating counterclockwise and the gas on the left is moving toward us at almost 1/3 the velocity of light. The iron line emission extending down to 4 keV comes from gas in the disk that is not moving toward us. Its redshift is due to a combination of its high velocity and gravitational redshift (see Lesson 7) due to the fact that it is emitted so close to the black hole (about 3 times the Schwarzschild radius). The red curve is a theoretical model for the line profile that would be emitted by such a system. The fact that it agrees well with the data is consistent with the idea that the luminosity of Seyfert galaxies comes from gas in a disk near a supermassive black hole. For more details, see Probing Massive Black Holes with X-ray Observations, by Chris Reynolds.

If you want to see a great little movie illustrating an X-ray flare near a black hole, click here.

If we see an active galaxy nearly edge-on, we won't see the direct X-rays because they will be shadowed by the disk. However, there will be plenty of turbulent gas all around the black hole. The gas that is above the plane of the disk will be illuminated by these X-rays and scatter some in our direction, so we will always see some X-rays from AGNs.

Optical emission: Gas somewhat further from the center of an active galaxy will absorb of the X-rays that shine on it. The X-rays will heat and ionize the gas, causing it to emit optical and ultraviolet emission lines that we see in the spectra of quasars and Seyfert galaxies. If we can see near to the center of the active galaxy, we will see broad emission lines due to gas moving with velocities of several thousands of km/s. Such galaxies are quasars and Type I Seyfert galaxies. (Quasars are probably the same phenomenon as Seyfert galaxies, except that they are more luminous.)

But if we see an active galaxy nearly edge-on, our view of the fast-moving gas near the central black hole will be blocked. But we might still see emission lines from slower-moving gas further from the center. Such galaxies are Type II Seyfert galaxies: active galaxies with narrow emission lines. So, it seems, Type I and Type II Seyfert galaxies are really the same phenomena. The only reason they appear different is that we are viewing them from a different aspect.

In blazars, one of the beams points almost directly at us. In that case, the radio, optical, and X-ray continuum radiation from the relativistic particles in the beam is so powerful that we don't notice the weaker emission line radiation produced by gas near the AGN.

Radio emission: Most of the radio emission from active galaxies comes from the beams of electrons going in the polar directions. We don't see much radio emission from most quasars and Seyfert galaxies because the beams are not pointed in our direction. DRAGNs are an exception to this rule: we see them because the beams are colliding with intergalactic gas, causing a buildup of magnetic fields and enhanced synchrotron emission in all directions at the lobes where the collision takes place.

(Return to course home page)
Last modified April 2, 2002
Copyright by Richard McCray