Quasars

What Are Quasars?

A quasar (contraction of QUASi-stellAR radio sources) is an astronomical object that looks like a star in optical telescopes (i.e. it is a point source), and has a very high redshift. The general consensus is that this high redshift is cosmological, the result of Hubble's law, which implies that quasars must be very distant and must emit more energy than dozens of normal galaxies.

Some quasars display rapid changes in luminosity, which implies that they are small (an object cannot change faster than the time it takes light to travel from one end to the other). The highest redshift currently known for a quasar is 6.4.

 

Properties of Quasars

Of the several hundred quasars observed, all spectra have shown considerable redshifts, ranging from 0.06 to the recent maximum of 6.4. Therefore, all known quasars lie at great distances from us, the closest being 240 Mpc away and the farthest being 5500 Mpc away. Most quasars are known to lie above 1000 Mpc in distance; since light takes such a long time to cover these great distances, we are seeing quasars as they existed long ago�the universe as it was in the distant past.

Although faint when seen optically, their high redshift at great distance imply that quasars are the brightest objects in the known universe. The currently brightest known quasar is the ultraluminous 3C 273 in the constellation of Virgo. It has an average apparent magnitude of 12.8 (when observing with a telescope), but it has an absolute magnitude of −26.7. So from a distance of 10 parsecs, this object would shine in the sky about as bright as our sun. This quasar's luminosity is, therefore, about 2 trillion (10��) times that of our sun, or about 100 times that of the total light of average giant galaxies like our Milky Way.

The hyperluminous Quasar APM 08279+5255 was, when discovered in 1998, given an absolute magnitude of −32.2, although high resolution imaging with the Hubble Space Telescope and the 10m Keck Telescope reveal that this system is gravitationally lensed. A study of the gravitational lensing in this system suggests that it has been magnified by a factor of ~10. It is still substantially more luminous than nearby quasars such as 3C 273. HS 1946+7658 was thought to have an absolute magnitude of −30.3, but this too was magnified by the gravitational lensing effect.

Quasars are found to vary in luminosity in differing time periods. Some vary in brightness every few months, weeks, days, or hours. This recent evidence has allowed scientists to theorize that quasars exhibit energy in a very small region, since each part of the quasar would have to be in contact with other parts on such a timescale to coordinate the luminosity variations. As such, a quasar varying on the time scale of a few weeks cannot be larger than a few light weeks across.

When compared to active galaxies, quasars exhibit many of the same properties. Radiation is nonthermal and some are shown to have emission jets and lobes. Quasars can be observed in many parts of the electromagnetic spectrum including radio, infrared, optical, ultraviolet, X-ray and even gamma rays while most quasars are found to emit in the infrared.
 

Quasar Emission Generation

Since quasars exhibit properties of all active galaxies, many scientists have compared the emissions from quasars to those of small active galaxies due to their likeness. The best explanation for quasars is that they are powered by supermassive black holes. Scientists theorize to create the luminosity of 1040 W (average brightness of a quasar), a super-massive black hole would have to consume the material equivalent of 10 stars per year. The brightest known quasars are thought to devour 1000 solar masses of material every year. Quasars are thought to 'turn on' and off depending on their surroundings. One implication is that a quasar would not, for example, continue to feed at that rate for 10 billion years, which nicely explains why there are no nearby quasars. In this framework, after a quasar finishes eating up gas and dust, it becomes an ordinary galaxy.

Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest quasars (z > 4) display a Gunn-Peterson trough and clearly have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region but rather their spectra contain a spiky area known as the Lyman-alpha forest. This indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.

One other interesting characteristic of quasars is that they show evidence of elements heavier than helium. This is taken to mean that galaxies underwent a massive phase of star formation creating population III stars between the time of the Big Bang and the first observed quasars. If no evidence for such stars is found and alternate mechanisms for producing heavy elements cannot be found, this may seriously undermine our current views of the universe. Light from these stars may have been observed using NASA's Spitzer Space Telescope, although as of late 2005 this interpretation remains to be confirmed.

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