What are they?
For us to see them from such great distances, quasars must be producing enormous energy -- they are about 1000 times brighter than an average galaxy! And, to make them even more amazing, the energy originates in a region smaller than a single star! Physicists had not thought about mechanisms to produce such energy from such a small volume until they were confronted with quasars.
One of their choices for the energy source of quasars was a black hole. As matter falls into a black hole, it is squeezed in such a way that the friction between in-falling particles makes the matter hot; light is emitted from this heated material, and the hotter the matter gets, the higher the frequency of light emitted. As analogy, consider what happens at the entrance to a cinema: If movie-goers do not queue properly, a crowd of people forms as all try to pass through the narrow doors. The heat in the crowd rises first due to the proximity of one person to another and second because of the friction between the bodies. On the one hand, it may seem difficult to think that mere friction between bits of matter could account for the energy of a quasar, but, on the other hand, gravity is unimaginably strong near a black hole.
In fact, the black hole solution to the quasar energy problem is the simplest one. Some researchers were somewhat skeptical about black holes altogether (although there is very much indirect evidence for their existence, no black hole has been really seen in a conclusive manner) and tried to find an alternative explanation for quasars. In one such explanation, it was proposed that a large number of supernovae occurring simultaneously over a very long time (as long as a quasar shines) can produce quasars' observed properties. At the present time, it seems that perhaps a combination of both explanations may be acceptable. A black hole in the center could be responsible for producing the frictional energy as well as for triggering a large number of supernovae. Let us now have a closer look at the surroundings of that black hole in the center. How does matter actually rush towards the black hole? The key word to answer this is accretion. It is nothing else than the accumulation of mass onto an object from its surroundings. Like a rolling snowball. But there is something special about astrophysical accretion, something that makes it different from rolling a bigger snowball.
Accretion in outer space tends to take place in a disk-like structure that surrounds the "accreting" object. Does this not remind you of the Solar System? Indeed, the most accepted theory on how the Solar System formed includes the process of accretion of matter onto the dense pocket of matter that eventually formed the Sun. Now you may think that in the case of quasars and their central black holes the same physical process may take place, but scaled up a lot. Well, this is not exactly true: Black holes are very small objects, and, as I said, the energy of a quasar comes from a region which may be smaller than a single star. What is the difference then? It is the density, and, therefore, the gravitational pull due to the black hole which makes the difference - remember, gravity is not related to size, but to mass and density!
The Unified Model for Active Galactic Nuclei
One might ask oneself, does the infalling matter near a black hole not hide the region where the energy forms? How, then, can we see quasars shine so bright? It is true, in fact, that there is matter that hides the quasar's bright center. Before actually getting into the relatively small accretion disk, matter gathers in a large, not-quite-flat "doughnut" of material surrounding the black hole. Thus, depending on one's viewing angle, this torus -- astronomers call it this rather than "doughnut" -- may hide the direct sight of the energy-producing accretion disk.
How does a quasar appear to us if we cannot see the accretion disk surrounding its black hole? The answer to this question is part of what makes the whole issue of quasars very interesting: Throughout the modern astronomical age, we have observed many objects in the Universe that have defied us in our attempts to classify them. The modern quasar model offers a quite natural explanation for these seemingly disparate objects. Striking things like radio galaxies and blazars can be explained as the very same objects as quasars, but being viewed from different angles.
Such an explanatory power makes a theory very attractive to scientists, and they tend to believe such theories as true very quickly - too quickly sometimes! In any case, at the present time, this accreting-black-hole model is accepted as an explanation for the behavior of quasars, radio galaxies, and blazars, known collectively as active galactic nuclei (AGNs), since they are thought to be always in the nucleus of a galaxy. And such galaxies are part of a class known as "active galaxies."
Galactic Nuclei: Old Quasars?
One question remains unanswered by astronomers: Because we see quasars exclusively at great distances -- and, therefore, the light we detect from them comes from a long time ago -- does this mean that all galaxies have had an AGN in their center at some stage, or is it the case that only a few of them did, and only during a short time in the past? Try to build your own theory on this by first obtaining information on the relative numbers of normal galaxies and quasars at high redshift and nearby.
Lenses through the HST. Two examples of gravitational lensing captured by the Hubble Space Telescope: HST 14164+5215 (left) is a pair of faint lensed images on either side of a brighter galaxy, while HST 15433+5352 (right) is a lensed source visible in this image as an extended arc about the elliptical lensing galaxy. Images courtesy of K. Ratnatunga (Carnegie Mellon Univ.) and NASA.
Erik Stengler studied physics in Cologne, Germany, and completed a M.Phil. and a Ph.D. in astronomy at Cambridge University in the United Kingdom. After several years of research in this field, he started a second Ph.D. in science didactics at the University of La Laguna in Spain and recently joined the team in charge of creating the new Interactive Science Museum in San Sebastian, Spain, to be opened in Spring 2000. He can be reached via email at firstname.lastname@example.org.
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