A quasar (contraction of QUASi-stellAR radio source) is an extremely bright and distant active nucleus of a young galaxy. Quasars were first identified as high-redshift sources of electromagnetic energy, including radio waves and visible light. They are point-like, similar to stars, rather than extended sources of light, such as galaxies. Although there was some initial controversy over the nature of these objects, the current scientific consensus is that a quasar is a compact halo of matter surrounding the central supermassive black hole of a young galaxy.
Quasars display a very high redshift, indicating that quasars and the Earth are rapidly moving away from one another, as part of the process of the expansion of the universe. When combined with Hubble's law, the implication is that quasars are very distant. To be observable at that distance, the energy output of quasars must dwarf that of almost every known astrophysical phenomenon in a galaxy, except for comparatively short-lived events like supernovae and gamma-ray bursts. Quasars may readily release energy in levels equal to the output of hundreds of average galaxies combined. The output of light is equivalent to one trillion Suns.
In optical telescopes, quasars look like single points of light (i.e. point source) although many have had their "host galaxies" identified. The galaxies themselves are often too dim to be seen with all but the largest telescopes. Most quasars cannot be seen with small telescopes, but 3C 273, with an average apparent magnitude of 12.9, is an exception. At a distance of 2.44 billion light years, it is one of the most distant objects directly observable with amateur equipment.
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 quasar J1819+3845 for another explanation). The highest redshift currently known for a quasar is 6.4.
Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies. No other currently known mechanism appears able to explain the vast energy output and rapid variability.
Knowledge of quasars is advancing rapidly. As recently as the 1980s, there was no clear consensus as to their origin.
The first quasars were discovered with radio telescopes in the late 1950s. Many were recorded as radio sources with no corresponding visible object. Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size. Hundreds of these objects were recorded by 1960, and published in the Third Cambridge Catalogue as astronomers scanned the skies for the optical counterparts. In 1960, radio source 3C 48 was finally tied to an optical object. Astronomers detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum. Containing many unknown broad emission lines, the anomalous spectrum defied interpretation—a claim by John Bolton of a large redshift was not generally accepted.
In 1962, a breakthrough was achieved. Another radio source, 3C 273, was predicted to undergo five occultations by the moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to optically identify the object and obtain an optical spectrum using the 200-inch Hale Telescope on Mount Palomar. This spectrum revealed the same strange emission lines. Schmidt realized that these were actually spectral lines of hydrogen redshifted at the rate of 15.8 percent. This discovery showed that 3C 273 was receding at a rate of 47,000 km/s. This discovery revolutionized quasar observation and allowed other astronomers to find redshifts from the emission lines from other radio sources. As predicted earlier by Bolton, 3C 48 was found to have a redshift of 37 percent the speed of light.
The term quasar was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in 1964, in Physics Today, to describe these puzzling objects:
So far, the clumsily long name "quasi-stellar radio sources" is used to describe these objects. Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form "quasar" will be used throughout this paper.
Later, it was found that not all (actually only 10 percent or so) quasars have strong radio emission (or, are "radio-loud"). Hence the name "QSO" (quasi-stellar object) is used (in addition to quasar) to refer to these objects, including the radio-loud and the radio-quiet classes.
One great topic of debate during the 1960s was whether quasars were nearby objects or distant objects as implied by their redshift. It was suggested, for example, that the redshift of quasars was not due to the expansion of space but rather to light escaping a deep gravitational well. However a star of sufficient mass to form such a well would be unstable and in excess of the Hayashi limit. Quasars also show unusual spectral emission lines which were previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well. There were also serious concerns regarding the idea of cosmologically distant quasars. One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including nuclear fusion. At this time, there were some suggestions that quasars were made of some hitherto unknown form of stable antimatter, and that this might account for their brightness. Others speculated that quasars were a white hole end of a wormhole. However, when accretion disc energy-production mechanisms were successfully modeled in the 1970s, the argument that quasars were too luminous became moot and today the cosmological distance of quasars is accepted by almost all researchers.
In 1979, the gravitational lens effect predicted by Einstein's General Theory of Relativity was confirmed observationally for the first time with images of the double quasar 0957+561.
In the 1980s, unified models were developed in which quasars were classified as a particular kind of active galaxy, and a general consensus emerged that in many cases it is simply the viewing angle that distinguishes them from other classes, such as blazars and radio galaxies. The huge luminosity of quasars results from the accretion discs of central supermassive black holes, which can convert on the order of 10 percent of the mass of an object into energy, as compared to 0.7 percent for the p-p chain nuclear fusion process that dominates the energy production in sun-like stars.
This mechanism also explains why quasars were more common in the early universe, as this energy production ends when the supermassive black hole consumes all of the gas and dust near it. This means that it is possible that most galaxies, including earth's native Milky Way, have gone through an active stage (appearing as a quasar or some other class of active galaxy depending on black hole mass and accretion rate) and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.
More than 100,000 quasars are known. All observed 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 earth, the closest being 240 Mpc (780 million ly) away and the farthest being 4 Gpc (13 billion ly) away. Most quasars are known to lie above 1.0 Gpc in distance; since light takes such a long time to cover these great distances, observers on earth 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 implies that these objects lie at a great distance from earth, making quasars the most luminous objects in the known universe. The quasar which appears brightest in our sky is the ultraluminous 3C 273 in the constellation of Virgo. It has an average apparent magnitude of 12.8 (bright enough to be seen through a small telescope), but it has an absolute magnitude of −26.7. So, from a distance of 10 parsecs (about 33 light-years), this object would shine in the sky about as brightly as the Sun. This quasar's luminosity is, therefore, about 2 trillion (2 × 1012) times that of the Sun, or about 100 times that of the total light of average giant galaxies like the 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 10 m Keck Telescope revealed 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 on a variety of time scales. Some vary in brightness every few months, weeks, days, or hours. This evidence has allowed scientists to theorize that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale 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.
Quasars exhibit many of the same properties as active galaxies: Radiation is nonthermal and some are observed to have jets and lobes like those of radio galaxies. Quasars can be observed in many parts of the electromagnetic spectrum including radio, infrared, optical, ultraviolet, X-ray, and even gamma rays. Most quasars are brightest in their rest-frame, near-ultraviolet (near the 1216 angstrom (121.6 nm) Lyman-alpha emission line of hydrogen), but due to the tremendous redshifts of these sources, that peak luminosity has been observed as far to the red as 9000 angstroms (900 nm or 0.9 µm), in the near infrared.
Iron Quasars show strong emission lines resulting from ionized iron, such as IRAS 18508-7815.
Since quasars exhibit properties common to all active galaxies, the emissions from quasars can be readily compared to those of small active galaxies powered by supermassive black holes. To create a luminosity of 1040 W (the typical 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 devour 1000 solar masses of material every year. Quasars turn on and off depending on their surroundings, and since quasars cannot continue to feed at high rates for 10 billion years, after a quasar finishes accreting the surrounding 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 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, indicating 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. Light from these stars may have been observed in 2005, using NASA's Spitzer Space Telescope, although this observation remains to be confirmed.
All links retrieved June 19, 2015.
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