HST image of 3C 273. Note the starlike
|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.
Discovery spectrum of 3C 273. Note the redshift of the hydrogen lines in the source relative to the
When Maarten Schmidt measured the spectrum of 3C 273 in 1964 it was the most distant object known… for about 15 minutes! Then his
|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.