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Quasars

3C273 Image

HST image of 3C 273. Note the starlike appearance. Note also the jet of material being ejected toward the lower right.

In the late 1950s radio astronomers began compiling catalogs of radio sources. At that time there were quite a number of radio sources which were not associated with previously known optical objects. The primary difficulty was that radio telescopes had poor resolution and large beam sizes so that it was often not possible to identify which of as many as several hundred faint stars was actually associated with a strong radio source. (Note that today this would not be a problem due to the advancements in resolution brought about by radio interferometric techniques). In 1960 one unassociated radio source (3C 48) was finally identified with a 16th magnitude stellar object. The identification was made when the moon occulted the radio source. Since this relatively strong radio source was associated with an object that appeared stellar, this category of objects became known as quasi-stellar radio sources (QSRS). This is the origin of the name quasar.

3C 273 Spectrum

Discovery spectrum of 3C 273. Note the redshift of the hydrogen lines in the source relative to the comparison spectrum. The wavelengths of H-Delta, H-Gamma and H-Beta are, respectively, 410, 434 and 486 nm.

When Maarten Schmidt measured the spectrum of 3C 273 in 1964 it was the most distant object known... for about 15 minutes! Then his colleagues in the next office measured the spectrum of 3C 48, which gave z = 0.36.

When a spectrum of this faint source was obtained it was observed to have strong broad emission lines which could not be identified with emission lines of known chemical elements. In 1963 the strong radio source 3C 273 was also identified with an inconspicuous 13th magnitude "star." This object too exhibited strong emission lines which could not be identified. Eventually a sequence of emission lines similar to the Balmer series of hydrogen was recognized in the spectrum of 3C 273, but shifted far to the red. The red shift observed for 3C 273, z = 0.158, was greater than the highest known redshift for a galaxy. However, assuming this redshift, the remaining emission lines could readily be identified with the standard emission lines known. Because the emission lines identified in this way are the same lines known to be present in nearby galaxies, and because galaxies are the only objects known to have systematic large redshifts, quasars are generally believed to be extragalactic.

If quasars are assumed to follow the Hubble law for distances, the the large redshifts must correspond with large distances. Indeed, at the large distances implied by some quasar redshifts, a normal galaxy would be too faint to be observed. Thus, quasars would appear to be hundreds of times brighter than normal galaxies.

Quasars have now been observed with redshifts ranging from 0.06 to above 6.0. However, there are no nearby quasars, and the number of quasars increases with redshift. Since redshift is related to distance, objects with large redshifts are farther from us; it has taken their light longer to reach us, and so we see them from a time farther into the past. Since quasars are more numerous at high redshift, they must also have been more common when the universe was younger. Quasars have either disappeared during the present time or have changed their characteristics and evolved into less luminous objects. According to current thinking on the subject, all large galaxies went through an active "quasar" stage early in their existence, but have now settled down into quiet middle age. In support of this idea is the growing evidence that all large galaxies contain supermassive black holes at their cores... the same type of object thought to power quasars and other AGN.


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This page was last modified on Friday 30th January 2009 @ 14:43pm

Science Mission Directorate Universe Division

Responsible SSU Personnel:

Dr. Kevin McLin (mclin at universe dot sonoma dot edu)

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