Quasars

Since we can see them despite their distance, they 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 quasar currently known is 6.4 ([1]). The maximum distance of observed quasars is significant because more distant quasars should easily be observable if they existed, this is taken to mean that the oldest observed quasars correspond to the beginning of galaxy formation.

Quasars also provide some clues as to the end of reionization. The oldest quasars 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 a spiky area known as the Lyman-alpha forest. This indicates that the intergalactic medium has reionized 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.

The first quasars were discovered with radio telescopes in the late 1950s; the first spectrum of a quasar, confirming its extragalactic nature, was obtained by Schmidt in 1963. Once they were identified it was possible to find them recorded in photographic plates dating back to the 19th century. Later it was found that not all (actually only 10% or so) quasars have strong radio emission (are 'radio-loud'). The name 'QSO' (quasi-stellar object) is sometimes given to the radio-quiet class. Other people talk about 'radio-loud' and 'radio-quiet quasars'.

One great topic of debate during the 1960's was whether quasars are nearby objects or distant objects as implied by their redshift. The strongest argument against a cosmologically distant quasars was that it implied energies that were far in excess of known energy conversion processes include nuclear fusion. At this time, there were some suggestions that quasars were made of antimatter and this accounts for their brightness. This objection was removed with the proposal of the accretion disc mechanism in the 1970's, and today the cosmological distance of quasars is accepted by almost all researchers, although a few insist, based largely on statistical arguments, that quasars are close by. [2]

In the 1980's, unified models were developed in which quasars were viewed as simply a class of active galaxies, and a general consensus has 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 is believed to be a result of friction caused by gas and dust falling into the accretion disks of supermassive black holes, which can convert about half of the mass of an object into energy as compared to a few percent for nuclear fusion processes.

This mechanism is also believed to explain 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 our own Milky Way, have gone through a quasar stage and are now quisicent because they lack a supply of matter to feed into their central black holes to generate radiation.