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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; but see J1819+3845 for another explanation). The highest redshift currently known for a quasar is 6.4 [1], which is significant because it implies a maximum distance—more distant quasars should be easily observable if they existed. This is taken to mean that the oldest observed quasars correspond to the beginning of galaxy formation. The first quasars marked the end of the dark age, the period after the emission of the cosmic microwave background radiation where there are no observable sources of radiation.
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 and great distance imply that quasars are the brighest objects in the known universe. In a general sense, quasars range in luminosity from 1038 W (the luminosity of the brightest radio galaxy) to 1042 W. The average quasar is found to be 1040 W, which is comparable to 1000 Milky Way galaxies, or 10 trillion Suns.
When compared to active galaxies, quasars exhibit much 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.
Quasars are also 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.
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 10^40 W (average brightness of a quasar), a super-massive black hole would have to consume 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, normal galaxy.
Quasars also provide some clues as to the end of the Big BangIn astrophysics, the term Big Bang is used both in a narrow sense to refer to the interval of time roughly 13. 7 billion years ago when the photons observed in the microwave cosmic background radiation acquired their black-body form, and in a more general's reionizationReionization is a process that occurs after the epoch of galaxy formation begins, and is the second of two major phase changes of hydrogen gas in the universe. The first is recombination, happening at a redshift (400,000 years after the big bang), at whic. The oldest quasars (z > 4) display a Gunn-Peterson troughAn effect seen in the spectra of quasars with redshift greater than 6 that is explained as due to absorption of quasar emission due to neutral hydrogen (hydrogen atoms). The effect exists because intergalactic hydrogen should absorb photons with energies 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 forestThe Lyman alpha Forest is the sum of absorption lines seen in spectra of distant galaxies and quasars, beginning from the Lyman alpha line at 121. 6 nm to shorter wavelength (higher photon energies). These absorption lines result from intergalactic gas th. 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 starsPopulation III stars are a hypothetical population and thus far unobserved of extremely massive stars that were believed to be formed in the early universe. They have not been observed directly, but are believed to be components of faint blue galaxies. between the time of the Big BangIn astrophysics, the term Big Bang is used both in a narrow sense to refer to the interval of time roughly 13. 7 billion years ago when the photons observed in the microwave cosmic background radiation acquired their black-body form, and in a more general and the first observed quasars. However, this prediction has the problem in that, as of 2004, no evidence for such stars have been found, and it may seriously undermine our current views of the universe if no such stars are found in the next few years, and alternate mechanisms for producing heavy elements cannot be found.
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