The Astrophysics Spectator



Interactive Pages


Other Pages


General Relativity

Black Holes in Galaxies with Active Nuclei

Many distant galaxies have extremely bright centers. These Active Galactic Nuclei (AGN) can outshine their host galaxies. Some bright AGNs are so distant that the host galaxy is invisible, and only the bright, star-like nucleus is seen; such objects are call quasars, which is derived from the term “quasi-stellar object.” Less brilliant and closer AGNs appear as galaxies with unusually bright nuclei; these galaxies are called Seyfert galaxies. Some galaxies with active nuclei shoot out jets of matter moving at close to the speed of light. These jets, which can extends tens of kiloparsecs from their origin, are seen by their radio emission. We are apparently looking directly into the jet of some of these galaxies; these AGNs, called blazars, and they vary violently and rapidly in brightness.

Despite this wide variety of appearance, the dominant theory is that all of these objects are a manifestations of a single object: a massive black hole at the center of a galaxy. This theory preceded the evidence that our own Galaxy harbors a massive black hole candidate in the Sagitarii A complex. In fact, the black hole theory for AGNs prompted the suggestion that our own galaxy harbors a massive black hole long before evidence for it appeared. The postulate that a black hole is the engine of a AGN is motivated by the rapidly changes in brightness of these objects: an AGN can change its power output in less than a day, which suggests that the source of the power is less than 200 AU across.

The basic AGN theory is that a massive black hole sitting at the center of a galaxy pulls gas onto itself, releasing gravitational potential energy that is either radiated away as light or converted into the kinetic energy of a jet. The light comes from a large accretion disk orbiting the black hole. This disk of gas orbiting the black hole slowly converts its gravitational potential energy into thermal energy, causing the gas to slowly drop into lower orbits, until eventually it falls inside the last stable orbit and onto the black hole. The energy released is radiated away from the disk's photosphere with electromagnetic frequencies spanning from the visible to the gamma-ray. The outer regions of the accretion disk generate optical and ultraviolet radiation, while the inner regions generate x-rays and gamma-rays. Some of this radiation drives a wind from the disk's photosphere, providing the mass for the jet. The jet itself may acquire energy directly from the radiation from the disk, or from magnetic fields generated and expelled by the accretion disk. Around this system orbits clouds of gas. This gas is absorbs and reradiates light from the accretion disk. These clouds produce strong emission lines at optical and ultraviolet frequencies.

The accretion disk of a bright AGN can radiate 100 trillion times the power of the Sun. Only 107 solar masses of gas per year need pass through the accretion disk onto the black hole to create this amount of energy.

How massive is the black hole driving all of this? It can be from one million to one billion times the mass of the Sun. A rough estimated of the mass can be derived from the fact that many AGNs drive a wind away from the black hole. If this wind is driven by radiation, then the force of the radiation, which is proportional to the energy released by the AGN, is larger than the force of gravity, which is proportional to the mass of the black hole. Setting these forces in equilibrium gives us an estimate of the mass of the black hole candidate. For an AGN radiating 100 trillion times the power of the Sun, the mass estimate is one billion solar masses.

A better measure of the mass of the black hole candidate is derived by combining measurements of variability of the radiation with measurements of the width of spectral lines. The idea is simple enough: if we can measure both the velocity and the distance of something orbiting the black hole, we can directly derive the mass of the black hole. With Sagittarii A* of our own Galaxy, the stars of the Sagittarii A complex are the orbiting objects. With AGNs, the orbiting objects are the clouds that create the line radiation.

Velocities for the clouds is found by measuring the shape of the emission lines from an AGN. The motion of the clouds Doppler shifts the radiation, with clouds moving towards us creating blue-shifted lines, and clouds moving away creating red-shifted lines. Seen together, all of these Doppler-shifted lines blend together to create a single broad line. The width of the line therefore gives a a measure of velocity.

The measure of distance comes from a characteristic of our theory. The light emitted by the accretion disk smoothly spans a broad range of frequencies. For this reason, this light is called continuum radiation. In the theory, this light is responsible for heating the orbiting clouds. If the accretion disk becomes brighter, the clouds should become hotter, and the line radiation they produce should become brighter. But because the clouds are far from the accretion disk, there is a time delay between the brightening of the accretion disk and the brightening of the line radiation. This delay is observed, and it gives us a measure of the distance from the black hole to the clouds.

Astronomers have been able to use this method to derive masses for several black holes candidates. For instance, the galaxy NGC 4151 is found to contain a black hole candidates of 10 million solar masses. The galaxy NGC 5548 is found to contain a black hole candidate of between 20 and 80 million solar masses. These are typical of the masses being derived. The black hole candidates in AGNs have so far been larger than our own Galaxy's Sgr A*, with its mass of 2.6 million solar masses.

The black hole in the AGN theory is the engine that lights the accretion disk and drives the relativistic jets, but otherwise it is invisible to us. None of the unusual effects of a black hole are necessary to produce what we see. The theory does not depend on general relativity being the correct theory of gravity. The theory only depends on the existence of a compact object of millions to billions of solar masses. This means that the most brilliant and common manifestation of the black hole tells us nothing about black holes, and everything about how energy is released by massive amounts of gas falling into a deep gravitational potential.

Ad image for The Astrophysics Spectator.