Also found in: Dictionary, Thesaurus, Acronyms, Wikipedia.
See H. L. Shipman, Black Holes, Quasars, and the Universe (2d ed. 1980).
quasar(kway -zar) A compact extragalactic object that looks like a point of light but emits more energy than a hundred supergiant galaxies. The name is a contraction of quasi-stellar object (QSO). Although they are bright optical sources, quasars emit most of their energy as infrared radiation. They are also strong X-ray sources. About 10% of quasars are also radio sources.
Quasars, of which several thousand are known, were discovered in 1963 as the optical counterparts to some powerful radio sources. The spectra of these objects were peculiar, with bright emission lines, apparently of an unknown element, superimposed on a continuum. Maarten Schmidt finally recognized the pattern of lines in one of these objects (3C 273) as the Balmer series of hydrogen redshifted to z = 0.158 (see redshift), showing that it was impossible for this to be a star within the Galaxy. Other quasars were then discovered to have far greater redshifts. The record is currently held by RD J030117+002025, whose redshift of 5.50 indicates that it is receding from us with more than 90% of the speed of light. Astronomers today interpret the quasar redshifts as Doppler shifts (see Doppler effect) arising from the expansion of the Universe, making them among the most distant and hence the youngest extragalactic objects we observe.
To be visible at such great distances, quasars must be exceedingly luminous: many have absolute magnitudes brighter than –27. There is however a great range in luminosity. In addition quasars themselves are often variable by a factor of two or greater, on timescales sometimes as short as a few hours. This indicates that their light-producing regions are sometimes less than a light-day across, which poses several problems in explaining their energy generation. The only process known to be efficient enough is accretion on to a supermassive black hole (of the order of 109 solar masses) that is located at the nucleus of the quasar. This is in agreement with the nonthermal continuum emission observed at all wavelengths.
Matter is accreted on the central black hole via an accretion disk, which is heated by the dissipation of gravitational energy to produce a thermal component of emission in the quasar spectrum known as the blue bump. Gas clouds located around this system are also irradiated by the black hole to produce low-ionization emission lines (such as the Balmer series of hydrogen); the line broadening reveals speeds in excess of 10 000 km s–1, indicative of the bulk motion of the clouds in orbit, accelerated by the central massive gravitational potential. This inner region of the quasar surrounding the black hole and its accretion disk is known as the broad-line region (BLR), and studies of the variability in the line emission show that it spans a region with radius typically of a few light-months. At larger radii of ten to a thousand light-years from the central ionization source, the gas clouds have more moderate speeds of a few hundred km s–1 and radiate strongly in forbidden lines of ionized metals as well as hydrogen. There is probably a continuous transition between the BLR and this narrow-line region (NLR). Both the broad and narrow emission lines are superimposed on the nonthermal spectrum from the central black hole.
Deep exposures of quasars reveal that the nucleus is located in a large but otherwise normal ‘host’ elliptical galaxy. The radio quasars have also been shown to lie at the center of clusters of galaxies for redshifts up to at least 0.7. Many quasars of both types are surrounded by emission-line nebulae extended over many tens of kiloparsecs in filaments and clouds. Some observations suggest that the host galaxy is distorted in shape and is partaking in an interaction that may supply fuel to the black hole. Alternatively, if a cooling flow is taking place in the galaxy clusters surrounding the radio quasars, these could be fueled by the mass deposition. Only one-tenth of a solar mass a year is required to power a luminous quasar.
There is a lack of quasars at very low redshift. The luminosity function of quasars show that they were most numerous around a redshift of 2 (when the Universe was half its present age) and their numbers have declined catastrophically since. The reason is not yet established.
The lines of sight to distant quasars pass through many foreground systems, some of which leave their imprint on the quasar spectrum in the form of absorption lines of ionized metals such as carbon and magnesium. Many of these narrow absorption lines are thought to be due to extended dark halos of ordinary galaxies. Some quasars show peculiar broadened line-of-sight absorption lines of Lyman-alpha that have a high column density known as damped Lyman-alpha systems. The foreground systems are most likely either high-redshift LSB galaxies, or the progenitors of present-day disk galaxies.
Most of the very highest redshift quasars show a large number of very narrow single absorption lines that completely ‘eat away’ the quasar continuum emission just blueward of the quasar's Lyman-α emission line. This is the Lyman-alpha forest and is caused by Lyman-α absorption by a population of small clouds spread over a large range in redshift. The small width of individual lines precludes their association with the quasar, and they are thought to arise instead from intergalactic shreds of primordial matter. The tendency of the number density of these Lyman-alpha clouds to decrease along the line of sight toward an individual quasar, is known as the proximity effect, and is caused by the quasar radiation ionizing any clouds too close to it.
Absorption features intrinsic to the quasar are found only in broad absorption line (BAL) quasars. The width of the lines implies that massive outflows of absorbing gas are taking place at speeds of tens of thousands of kilometers per second away from the quasar.