The first quasars were discovered as radio loud sources in the late 1950's & early 1960's. The first optical discovery to accompany a radio source came in 1960 when radio source 3C 48 was tied to an optical object. The optical spectrum was difficult to interpret with many before unseen emission and absorbtion lines. In 1963 another source was tied with an optical component and it too exhibited these strange emission /absorbtion lines. It was soon discovered that these weren't so strange as they were just redshifted.
Quasar Spectrum: Optical spectrum of the distant quasar 3C 273. Notice both the redward shifts and the widths of the three hydrogen spectral lines marked as H, H, and H. The redshift indicates the quasar°«s enormous distance. The width of the lines implies rapid internal motion within the quasar itself. (Note that in this figure, red is to the right and blue is to the left.) (Palomar/Caltech)
The high redshifts involved according to the optical spectra put these Q.S.O's(quasi stellar objects) at cosmological distances. Initially this proposal was opposed because of the energies involved, typically of the order of 10&sup40; W, if the objects were of cosmological origin. However today quasars are thought to be of cosmological origin with the power source originating from an accretion disc powering a massive black hole. This accretion process is efficent enough to convertabout half of the mass of an object into energy and is substantially more efficient than nuclear fusion. Hence quasars are found in the centre of galaxies, i.e. quasars are a special case of A.G.N. and even our own galaxy may have once contained a quasar! Quasars are thought to run out of fuel after a few billion years and hence are only seen at cosmological distances.
Quasars provide an excellent way in which to "illuminate" the intergalactic medium(IGM). The most common element in the universe is Hydrogen, and the distribution of intergalactic gas can be mapped by making use of the spectrum of Hydrogen. The most prominent line is the transition from n=2 to n=1 state in the hydrogen atom and is called the Ly-α emission line. The transition occurs at a wavelength of 121.6nm. As the radiation from the distant quasar passes through various intergalactic clouds the radiation is absorbed at different wavelenghts depending on the position of the clouds relative to the quasar. The reason for this is that the quasar is receeding from the cloud due to the expansion of the universe. Hence a spectrum is seen with a peaked emission line corresponding to the Ly-α emission line with a "forest" of absorbtion lines at shorter wavelengths.
In order to see an absorbtion spectrum at all, and not just an absorbtion trough(Gunn-Peterson trough), then the IGM must be ionized with only a small fraction of neutral hydrogen present which absorbs the radiation. Hence quasars can shed some light on how the IGM came to be reionized. In the spectra of some very distant quasars the Gunn-Peterson trough is seen which indicates that at that time the IGM was not yet ionized.