A quasi-stellar radio source ("quasar") is a very energetic and distant galaxy with an active galactic nucleus. They are the most luminous objects in the universe. Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies.
While there was initially some controversy over the nature of these objects—as recently as the early 1980s, there was no clear consensus as to their nature—there is now a scientific consensus that a quasar is a compact region in the center of a massive galaxy surrounding its central supermassive black hole. Its size is 10–10,000 times the Schwarzschild radius of the black hole. The quasar is powered by an accretion disc around the black hole.
Quasars and QSOs
These objects were named Quasistellar Radio Sources (meaning "star-like radio sources") which was soon contracted to quasars. Later, it was found that many similar objects did not emit radio waves. These were termed Quasistellar Objects or QSOs. Now, all of these are often termed quasars (Only about 1% of the quasars discovered to date have detectable radio emission).
Here are some Hubble Space Telescope quasar images, and the following figure shows the quasar 3C273, which was the first quasar discovered and is also the quasar with the greatest apparent brightness. It will be discussed further below.
Quasars Are Related to Active Galaxies
The quasars were deemed to be strange new phenomena, and initially there was considerable speculation that new laws of physics might have to be invented to account for the amount of energy that they produced. However, subsequent research has shown that the quasars are closely related to the active galaxies that have been studied at closer distances. We now believe quasars and active galaxies to be related phenomena, and that their energy output can be explained using the theory of general relativity. In that sense, the quasars are certainly strange, but perhaps are not completely new phenomena.
Quasar Redshifts Imply Enormous Distance and Energy Output
The quasars have very large redshifts, indicating by the Hubble law that they are at great distances. The fact that they are visible at such distances implies that they emit enormous amounts of energy and are certainly not stars. The following image from the Sloan Digital Sky Survey shows the three most distant quasars known. The quasars are the faint red smudges near the head of each arrow. Their redshift parameters are 4.75, 4.90, and 5.00 respectively, which places them at distances of about 15 billion light years (Ref).
The Energy Source of Quasars is Extremely Compact
Quasars are extremely luminous at all wavelengths and exhibit variability on timescales as little as hours, indicating that their enormous energy output originates in a very compact source. Here are some light curves at different wavelengths illustrating the variability in intensity of some quasars and other active galaxies. Here is an explanation of these light curves. In all cases, the timescale for variability of the light from an active galaxy sets an upper limit on the size of the compact energy source that powers the active galaxy. These limits are typically the size of the Solar System or smaller.
Some quasars emit radio frequency, but most (99%) are radio quiet. Careful observation shows faint jets coming from some quasars. The above images of the quasar 3C273 illustrate both a jet in the optical image on the left and radio frequency emission associated with the jet on the right. Here are some spectra of quasars and other active galaxies - see the following description.
Relationship of Quasars and Active Galaxies
The quasars are thought to be powered by supermassive rotating black holes at their centers. Because they are the most luminous objects known in the universe, they are the objects that have been observed at the greatest distances from us. The most distant are so far away that the light we see coming from them was produced when the Universe was only one tenth of its present age.
The present belief is that quasars are actually closely related to active galaxies such as Seyfert Galaxies or BL Lac objects in that they are very active galaxies with bright nuclei powered by enormous rotating black holes. However, because the quasars are at such large distances, it is difficult to see anything other than the bright nucleus of the active galaxy in their case. As we have noted above, modern observations have begun to detect around some quasars jets and evidence for the surrounding faint nebulosity of a galaxy-like object.
Evolution of Quasars
The standard theory is that quasars turn on when there is matter to feed their supermassive black hole engines at the center and turn off when there is no longer fuel for the black hole. Recent Hubble Space Telescope observations indicate that quasars can occur in galaxies that are interacting with each other. This suggests the possibility that quasars that have turned off because they have consumed the fuel available in the original galaxy may turn back on if the galaxy hosting the quasar interacts with another galaxy in such a way to make more matter available to the black hole.
Abundance of Quasars in the Early Universe
Looking at large distances in the Universe is equivalent to looking back in time because of the finite speed of light. Thus, the observation of quasars at large distances and their scarcity nearby implies that they were much more common in the early Universe than they are now, as illustrated in the adjacent figure (see the Source for a further discussion of the figure).
Hungry Black Holes
Notice that the greater abundance of quasars early in the Universe would be consistent with the mechanism discussed above whereby a quasar shuts off when its black hole engine has consumed the fuel available in the host galaxy. We would expect that generally in the early Universe there may have been more mass easily accessible to the black hole than later, after much of it had been consumed. Perhaps later quasars are more dependent on interactions between galaxies to disturb mass distributions and cause galaxies to begin to feed the hungry black hole.
