Power Source for Active Galaxies and Quasars

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So here's the problem: how does nature produce such objects that are luminous over a large range of wavelengths and generate the energy in a very small volume? The number of stars needed to produce the tremendous luminosity could not be packed into the small region and neither would they produce the peculiar non-thermal radiation. One mechanism you have already learned about is the intense radiation produced by hot gas in an accretion disk around a black hole.

In order to produce the enormous amount of energy seen in active galaxies and quasars, the black hole must be supermassive. The intense radiation from the disk would drive the gas outward if the black hole did not have enough gravity to keep the gas falling onto the black hole. In order to keep the gas spiralling in and heating up, the mass of the black hole must be hundreds of millions to several billion solar masses. The accretion disk is a few trillion kilometers across (a few light months across) but most of the intense radiation is produced within a couple of hundred billion kilometers from the black hole.

accretion disk around supermassive black hole

The gas in the disk is heated by the friction it experiences rubbing against other gas in the disk and also by the release of gravitational potential energy as it falls inward onto the black hole. If you have dropped down from a large height (from a tree or ladder?) you know that your feet absorb a lot more energy than if you dropped from a small height (you did land on your feet, hopefully). The gravitational potential energy you had above the ground was converted to kinetic energy (energy of motion) as you fell. If you dropped to the ground from a very high height (great gravitational potential energy), you know that you would hit the ground with a great amount of energy. The situation would be worse on an object with stronger gravity. The process of converting mass to energy from falling onto a black hole has an efficiency that is over ten times as large as the efficiency of nuclear fusion. The amount of mass needed to power the nuclei in active galaxies is from one to ten solar masses per year.

If there is a strong magnetic field in the accretion disk, the magnetic field lines can be distorted into a tangled, narrow mess of magnetic field lines that run toward the poles perpendicular to the accretion disk. Gas escaping along the magnetic field would produce the beams of electrons and gas seen in the jets to make the radio lobes of radio galaxies. Also, the shape of the accretion disk may play a role in directing the gas into the jets. The outer parts of the disk can be thick, but they will narrow down to a very thin layer just outside of the black hole. A thick accretion disk that narrows down only very close to the black hole could pinch the outflowing gas into a narrow beam.

Around the accretion disk are relatively dense clouds of hot gas that could be responsible for the broad emission lines seen in Type 1 Seyferts. Further out is a thick dusty molecular doughnut-shaped ring with a diameter of ten to several hundred light years. The particular type of active galaxy seen then simply depends on the angle the accretion disk and dust ring are to the line of sight.

If the accretion disk is tipped enough, the fast-moving hot clouds that produce the broad emission lines of Type 1 Seyferts are visible. If the dust ring hides the accretion disk, then only the slower-moving hot clouds that are farther from the black hole are visible. The results is the narrow emission lines and the dust ring glowing in the infrared of a Type 2 Seyfert. If the disk is face-on and a beam of radiation is being produced, then the active galaxy is a BL Lac object.

The Hubble Space Telescope has imaged the nuclei of several active galaxies. Around the core of the radio galaxy NGC 4261 is a ring of dust and gas about 400 light years in diameter and the jets emerge perpendicular to the plane of the dust/gas ring. The black event horizon of the supermassive black hole is too small to be resolved from our distance. Select the image to view an enlargement of the HST image in another window.

core of NGC4261

The core of the active galaxy M87 is seen to have a disk of hot gas moving very quickly around the center. Doppler shifts of the disk material close to the center show that the gas is moving at speeds of hundreds of kilometers per second. Blueshifted lines are produced from one part of the disk and redshifted lines are produced from the opposite part of the disk. This is clear proof that the disk is rotating. The speed and distance the gas is from the center show that the central object must have a mass of 2.5 billion solar masses. Only a black hole could be this massive and compact. The jet coming from the nucleus (visible in the wider-field view at right) is also seen to be perpendicular to the plane of the disk.

M87's accretion disk at its core

The Short Life of an Active Galaxy

Galaxy nuclei do not remain active for billions of years. Eventually, the gas fuel runs out in the accretion disk. The disk's gas can be replenished if some stars in the galaxy wander too close to the central black hole and get stretched apart by the black hole's strong gravity.

Select the link below to show a spiral galaxy turning into a quasar when some material is dumped onto the supermassive black hole at the center. It is a mpeg movie, so you will need to have a mpeg viewer.

Quasar movie from the Space Telescope Science Institute

Also, galaxy mergers and collisions will keep the gas and stars in the central part of active galaxies sufficiently stirred up so some of that material will become part of the accretion disk. The expansion of the universe decreases the rate at which interactions will happen. Because the frequency of galaxy close encounters decreases over time as the universe expands, the quasars and active galaxies can last for only a few billion years at most.

changing quasar number shows that universe changes over time

Quasars tend to be found at great distances from us; there are no nearby quasars. When we look at quasars, we see them as they were billions of years ago. The number of them increases at greater distances, so that must mean they were more common long ago. The number of quasars peaks at a time when the universe was about 20% of its current age. Back then the galaxies were closer together and collisions were more common than today. Also, the galaxies had more gas that had not been incorporated into stars yet. The number of quasars was hundreds of times greater than the time closer to the present. At very great distances the number of quasars drops off. The light from the most distant quasars are from a time in the universe before most of the galaxies had formed, so fewer quasars could be created.

This model predicts that there should be many dead quasars lurking at the cores of galaxies. Astronomers are beginning to find the inactive supermassive black holes in some galaxies. In most galaxies the central black hole would have been smaller than the billions of solar mass black holes for quasars. This is why the less energetic active galaxies are more common than quasars. Our galaxy harbors a supermassive black hole in its core that has a mass of ``only'' 2.5 million solar masses. Astronomers are studying the cores of other normal galaxies to see if there are any signs of supermassive black holes that are now ``dead''.

The beautiful grand-design spiral, the Whirlpool Galaxy
Courtesy of NOAO/AURA/NSF
core of M51
X marks the spot in the core of the Whirlpool Galaxy! The darkest bar may be the dust ring seen edge-on. The jet seen in wider fields of view is perpendicular to the darkest dust ring. The lighter bar may be another disk seen obliquely. A million solar mass black hole is thought lurk at the center.
Courtesy of Space Telescope Science Institute

An important implication of the fact that there were more quasars billions of years ago than there are now, is that the universe changes over time. The conditions long ago were more conducive to quasar activity than they are today. In the next chapter you will explore the overall evolution of the universe. You will need to remember this point about a changing universe when you consider ideas for how the universe formed and grew. Also, the sharp drop in the quasar number for the earliest times is evidence for a beginning to the universe.

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last updated: 17 June 2002

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel