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16.18 Quasars

In 1963, Cal tech astronomer Maarten Schmidt made the amazing discovery of objects with large red shifts. The recession velocities can be so high that they are a significant fraction of the velocity of light. These objects were originally called quasi-stellar radio sources or quasi-stellar objects (QSO) denoting the fact that they appeared almost star-like on photographic plates and can have weak radio emission. The general term quasar refers to a distant galaxy in which the light from an active nucleus completely swamps the light from its surroundings. Astronomers think that the quasar phenomenon is caused by a super massive black hole at the center of a galaxy. When the energy from close to the black hole outshines the surrounding galaxy, we see a quasar.


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Arp 220. Click here for original source URL.

The highest red shift quasar has roughly a value of z = 7 and most lie in the range z = 1 to z = 3. What are the implications of such large red shifts? If we assume that the red shift is caused by the expansion of the universe, then quasars must be very distant. The light travel time is very large, and we must be observing light emitted when the universe was much younger than it is now. Remember that a journey out into space is a journey back in time. Large telescopes can be used as time machines. We see distant galaxies and quasars as they were soon after their formation. Therefore, the study of the most distant objects brings us face to face with questions about the origin and large-scale structure of the universe itself. Also, if quasars are very distant, their luminosity or intrinsic brightness must be enormous. Assuming a cosmological red shift, 3C 273 is 620 Mpc, or 2 billion light-years distant, and shines with the light of 1014 times solar luminosity, or 100 trillion Suns! This is an almost unimaginable amount of energy. It is no wonder that the interpretation of quasar properties has sometimes been controversial.

There is a crucial distinction between galaxy red shifts and quasar red  shifts. Galaxies follow the Hubble relation, in which estimated distance is correlated with radial velocity. This correlation has a natural interpretation in terms red shift and assign it a distance. But quasars do not have any property that reliably indicates distance. For example, the absolute luminosity of quasars ranges over a factor of more than a thousand. This wide range means that the apparent brightness of any two quasars can differ by a factor of thousands at the same red shift. Quasar brightness is a very poor distance indicator. Also, quasar red shifts are far higher than the range over which the Hubble relation has been tested. The distance to a quasar is meaningful only in the context of a cosmological model. The enormous distance estimates are the result of the assumption of cosmological red shifts and a model that converts red shift into distance.

Is there any evidence to support the assumption of cosmological red shifts for quasars? Some astronomers have pointed out cases in which quasars with high red shifts appear to be associated on the sky with nearby galaxies that have low red shifts. If objects with different red shifts were shown to be physically connected or at the same distance, it would cast doubt on the assumption of cosmological red shifts. It all comes down to statistics. The sky is filled with thousands of quasars and hundreds of thousands of galaxies, so some alignments are bound to occur by chance.

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Quasar 1229+204 in the Core of a Colliding Galaxy. Click here for original source URL.

A large body of evidence now supports the idea that quasars are at the distances indicated by their cosmological red shifts. As their name implies, quasars are often not completely stellar in appearance. Images of quasars at red shifts from a range of z = 0.3 to 1 reveal fuzz, or nebulosity, surrounding the bright core. The nebulosity has the size and brightness expected of a luminous galaxy at that red shift. In addition, quasars often lie in clusters of galaxies at the same red shift; this association is strong circumstantial evidence that the quasars are at cosmological distances.

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Quasar MC2 1635+119. Click here for original source URL.

Perhaps the most convincing evidence that quasars are extremely distant comes from observations of gravitational Lansing. General relativity predicts that mass can bend light, which leads to the distorted image of a background object. Dark mass in galaxy clusters can distort the light of a background galaxy. About 1 in 300 quasars happens to lie directly behind an intervening galaxy. The Lansing causes the magnification of the quasar light and the formation of multiple images. Gravity can deflect all forms of electromagnetic radiation, so we can observe Lansing of radio waves as well as visible light. If the alignment is nearly perfect, an Einstein ring is seen. If the Lansing object is an elliptical galaxy, the quasar light usually splits into four images in a cross, with the Lansing galaxy in between them. The probability of finding four unrelated quasars so close together, with identical red shifts and spectra, is infinitesimally small. This type of configuration can only be produced by the effect of gravitational Lansing. The quasar thus must be more distant than the Lansing galaxy.

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Two quasars - the one on the right has a host galaxy while the one on the left does not. Click here for original source URL.

How are quasars related to other active galaxies? There is continuity in observed structure between normal galaxies, active galaxies at low red shift, and quasars at much higher red shift. Active nuclei are rare in normal galaxies, but a few nearby examples can be found. Searching larger volumes of space turns up even rarer but more luminous active nuclei, but at such large distances that the nebulosity from the surrounding galaxy is difficult to discern. At the greatest distances, only highly luminous objects can be seen, and the light from the surrounding galaxy is correspondingly weak. As red shift and distance increase, the brightness and angular size of a normal galaxy decrease. Also, the light from the active nucleus drowns out the starlight from the surrounding galaxy. By a red shift of z = 0.3 to 0.5, the host galaxy is barely visible. At this point, astronomers refer to the object as a quasar. The most distant quasars are too remote for the host galaxy to be resolved; they appear as point sources of intense light.