A guide to QUASARS.


What are Quasars?

Qusars give out radio waves. They were originally believed to be stars, until careful analysis revealed that their stellar appearance must be due to the fact that they are very distant. And so, they were called quasi-stellar radio sources, which was later abbreviated to simply Quasars.

As will be seen on this page, quasars are the cause of, and involved in, some of the stranger events in this universe.

Quasars are at the distances of galaxies, but are certainly not galaxies, as these contain stars and therefore have absorption lines, whereas quasar spectra are dominted by emission lines.

Below is a Gamma Ray image of the Quasar 3C 273. As can be seen in the image, it is one of the larger quasars, and is well defined, with its outer surface perimeter clearly shown in red. The small white dot at the bottom of the quasar, possibly represents another object (probably a star) close to the quasar.

If quasars obey the Hubble law and are at distances that correspond to their large redshifts (the velocities of recession of galaxies are proportional to their distances from us, and recession produces a Doppler shift of the galaxies' spectral lines towards longer, redder, wavelenghts-therefore, the higher the redshift of an object, the further away it lies), then they must be brighter than than the brightest galaxies, because of their distance.


General information on Quasars.

Quasars vary in their luminosity on various time scales, from months, to weeks, and sometimes to days.

It is because of their changes in luminosity on these time scales of months or even smaller amounts of time, that it is belived that the region where the energy is generated can be no larger than a few light months.

The actual variation in luminosity is irregular and eveidently, at random. Since quasars are highly luminous, a change in brightness means a large amount of energy is released rather suddenly. Because the fluctuations occur in such short times, the part of a quasar responsible for the light and radio variations must be smaller than the distance light travels in a month or so.


The problem with redshifts.

The difficulty faced by astronomers to account for this flood of energy released led some of them to suggest that the redshifts of the quasars are not due to the Doppler effect (if a light source is approaching or receding from the observer, the light waves will be, respectively crowded close together or spread out). The spectral lines in galaxies are shifted to red because the universe is expanding. These astronomers argued that rather than being a consequence of the expansion of the universe, the redshift of quasars was produced by some unknown mechanism.

If this hypothesis were correct, then the measured redshift could not be used to estimate the distances of quasars. At the time, there were no alternative methods for estimating their distances , the quasars could be assumed to be close enough to us so that their energy output was within the range of that associated with normal galaxies.

Some astronomers supported this point of view, and have sought evidence for physical associations between high-redshift quasars and low-redshift normal galaxies. If two objects are physically associated, they must be at the same distance . If they also have very different redshifts, then we would forced to conclude that redshifts are not always a reliable indicator of distance.

There are many cases in which quasars with large redshifts appear on the sky close to galaxies with small redshifts. It is always possible, however, that these are chance superpoitions of two objects that are really at very different distances. There remain too few examples of an apparent association between a quasar and a galaxy with discordant redshifts to convince most astronomers that the redshifts of quasars are not the result of the expansion of the universe.

Some astronomers have turned this arguement around and have searched for clusters of galaxies in the vicinity of quasars. If redshifts can be measured and distances derived for these normal types of objects, and if the redshifts turn out to be the same as that of the nearby quasar, then we would have compelling evidence that the quasar also obeys the Hubble law. This task is not easy observationally because normal galaxies are fainter than quasars, and as such, a more difficult to detect.

However, studies have shown that quasars are often surrounded by small clusters of galaxies, with these galaxies having the same redshift as the quasar. It is strongly believed that the these objects are physically associated.


A possible connection between Black Holes and Quasars.

Observations of quasars and various different types of galaxies that are unusually active emitters of optical, x-ray, and radio radiation suggest that what all of these objects have in common is a compact source of enormous energy, presumeably buired in the nucleus of an otherwise normal galaxy.

The most widely accepted reason is that quasars, and other types of active galaxies, derive their energy output from an enormous black hole at the center of a galaxy. This black hole must be very large, at least a billion solar masses. With such a large black hole, relatively modest amounts of additional material, about ten solar masses per year, falling into the black hole, would be adequate to produce as much energy as a thousand normal galaxies and could account for the total energy of a quasar.

Because a black hole can not radiate energy itself, the enrgy comes from material very close to the black hole as it attracts matter in the form of stars, dust, and gas, which is orbiting around in the dense nuclear regions of the galaxy. This material then spirals in towards the black hole, and forms an accretion disk of material around it. As the material spirals closer to the black hole, it accelerates and heats through compression, to millions of degrees. This superheated matter can radiate enormous amounts of energy as it falls into the black hole.

This arrangement explains a number of observed phenomena. First, it can produce the amount of energy that is actually observed to be emitted by quasars and active galactic nuclei. Second, since the black hole is also fairly compact in terms of its circumference, the emission produced by infalling matter comes from a small volume of space. This condition explains the fact that quasars vary on a time scale of weeks to months.


What causes Quasars to radiate?

The radio radiation from quasars is in the form of synchrotron radiation, which is when a charged particle enters a magnetic field, moves around the lines of force, speeds up close to the speed of light, and radiates energy. With quasars, this radiation produces emission not only at radio wavelengths, but also in the visible and x-ray regions of the spectrum.

The quasar emission lines, that do not come from the same region as the x-ray and synchrotron radiation, must originate from ionized gas at not too high a temperature. At the temperatures required to produce x-ray emission, the gas would be completely ionized and no atomic emission lines could be produced. The strengths of the emission lines vary on time scales of months, however, so they cannot be spread throughout the galaxy in which the quasar is found.

Models have suggested that the broad emission lines are formed in relatively dense clouds with half a light year of a black hole, with the broadening of the lines produced by the Doppler effect, but it is unknown as to whether the motions of the gas are caused by turbulence, rotation around the black hole, expansion, or contraction.

Quasars and other active galaxies emit jets that extend far beyond the limits of the parent galaxy. Observations show that these jets originate within 3-30 light years of the parent quasar or galactic nucleus.

Matter in the black hole's accretion disk is continually being depleted by falling into the black hole or being blown out from the galaxy in the form of jets. A quasar can only continue to radiate as long as there is gas available to replenish the accretion disk. Extra matter is needed.

One possibility as to where this matter comes from is that very dense star clusters form near the centers of galaxies. These stars might then supply the fuel, either through gas that is lost during the normal course of stellar evolution by means of stellar winds and supernovae explosions, or becuase the tidal forces exerted by the black hole are strong enough to tear the stars apart.

Another source of fuel might come from collisions of galaxies, where two galaxies collide and merge, with dust from one galaxy fuelling the black hole of the other galaxy

It has been observed that very bright quasars were much more common a few billion years ago than they are now. An explanation for newer quasars being darker is simply that they have used up all the possible fuel sources available to them. Many of the newer quasars have been found in galaxies that have recently been involved with a collison with another galaxy, showing that there is now no other source of fuel available to it, other than the energy given out by the black hole after two galaxies collide.


Faster than light velocities in Quasars.

In several quasars, small, discrete sources have been found that change position from one observation to the next. These motions are generally radially outward from the center of the quasar image. By measuring the time difference for the different positions of these sources, it has been calculated that if the redshifts of the quasars are cosmological, then some of the velocities of the moving sources are apparently in the range of five to ten times the speed of light.

Because such speeds are impossible, these results were taken as evidence that the quasars must be relatively nearby.

The most widely accepted explanation for these observations is that if an object is moving towards an observer at a direction other than straight on, it can appear to have a far greater speed than it really does, due to the finite speed of light.


Quasars acting as Gravitational Lenses.

Einstein's general theory of relativity predicts that light will be deflected in the vicinity of a strong gravitational field. Quasars can help to test this theory. If light from a distant quasar passes nearby an intervening galaxy on its way to Earth, then its light will be deflected by the galaxy's strong gravitational pull, and, with the galaxy acting as a sort of "gravitational lens", we may see two or more quasars when in fact there is only one. However, galaxies may not be the only gravitational lenses, with other possibilities including dark matter.

In 1979, astronomers at Arizona noticed that a pair of quasars, separated by only 6 arcsec, and known collectively as 0957 + 561, looked remarkably similar in appearance and spectra.

They are both on the order of about 17th magnitude, and they both have an equal redshift of 1.4. It was suggested by the astronomers, that the two quasars might actually be only one, and that we are seeing two images produced by an intervening object, one that is acting as a gravitational lens.

Now, however, it is known that there is an 18th magnitude galaxy lies in the same direction as one of the quasars. In fact, the galaxy is a member of a cluster of galaxies, which has a redshift of 0.39 and is thus much closer than the quasar. The geometry and estimated mass of the galaxy are correct to produce the gravitational lens effect.

There is also some convincing evidence for several other gravitational lenses, and the search is on to discover more.

This search is difficult, because if theoretical calculations are correct, the light from about only one quasar in a thousand will pass close enough to a galaxy so that the galaxy can act as a gravitational lens and produce a double image of the background quasar.

Gravitational lenses are not limited to producing double images, and can produce multiple images, and even arcs and rings. Images produced by point-like gravitational lenses can appear much brighter than the actual source would ordinarily appear to be in the absence of lensing.

Due to this, some of the brightest quasars may owe their apparently high luminosities to enhancement by gravitational lensing.

Written by Jon Talpur at Keele University, 1997.


Links

For further reading on Quasars, check out these sites:

Hubble Space Telecope pictures of Quasars

Quasars and the Lyman Forest

Radio to X-ray observations of Quasars

The Quasar FAQ


Bibliography

"Exploration Of The Universe" by George O. Abell, David Morrison & Sidney C. Wolff