Quasars: Definition

The term quasar is a shortened version of “quasi-stellar object”. This in turn is a shortened version of “quasi-stellar optical object coincident with a discrete radio source”. Whew! The term Quasi-stellar means star-like. Since the first few objects were discovered some have not been identified with a radio source. So currently the term quasar means an object that looks like but is not a star.

Quasars are now known to be the active centers of very young galaxies. They are believed to be related to Seyfert galaxies and BL Lacerta objects. All of these objects appear to be the result of a super-massive black hole at the center of a host galaxy. It is believed that Quasars are found in gas-rich spiral galaxies while BL Lacerta objects are found in gas-poor elliptical galaxies. In both cases the galaxies would themselves often be super-massive compared to ordinary galaxies. Seyfert galaxies are found in spiral galaxies and may simply occur in less massive (of the super-massive galaxies) than do Quasars. There are at least 200,000 Quasars catalogued today (2010).

Quasars: Description

Quasars may be the most energetic (and luminous) objects in the universe, at least 100 times as luminous as an average galaxy. However a quasar’s energy is mostly radiated at higher, non-visible, wavelengths. So a single quasar can release >10^52 ergs per year or 10^58 to10^60 ergs over the life of the quasar. That is 10,000 to one million times the energy output of an entire galaxy like our own!

The mechanism that creates the enormous energy is believed to be synchrotron radiation. This radiation is caused by a super-massive black hole in the galaxy center. Matter that falls into a black hole does not fall directly in; rather, it orbits the black hole in an accretion disk. As matter orbits it emits photons, as the gas gets hotter and hotter, it gives more energy to the departing photons, eventually giving off synchrotron radiation at extremely high energies (x-rays). This energy is produced much more efficiently than in fusion reactions so that it approaches the theoretical limit (E=mc^2), some 1000 times the energy produced by fusion. Assuming a 10^8 solar mass black hole of radius 10 AU, calculations show that it would take the equivalent of one solar mass per year falling into the black hole to create the needed energy for a Quasar with energy equal to 10^52 erg per year. This assumes ten percent efficiency. The center of the host galaxies, where mush matter is gather close together could certainly provide the needed mass.

This is one of several reasons that quasars have been such enigmatic objects. An average galaxy, like our own Milky Way is on the order of 100,000 to 500,000 light years across. The average quasar in on the order of a light-day to light-week across. Astronomers first faced with these facts found it very difficult to explain how such an object could possibly produce such energy. Fusion reactions could be responsible for that much output.

Common Quasar Characteristics


• Most QSOs are bright in the X-rays and Ultraviolet

• The optical light output of QSOs is a small fraction of their total energy output

• Most QSOs showed variability in their luminosity on timescales of a few minutes to hours --> this implies that the volume which is generating all the energy is about the size of our Solar System!

Quasars: Distribution

Most Quasars are found at a range of red-shift values from z=1.5 to z=3.5. This corresponds to about 5 to 10 billion light years distant. Quasars are all found, so far, at z=0.06 to z= 6.43. This would put them at about 750 million to just over 13 billion light years from Earth using the current best guess for the value of the Hubble Constant. Most are farther than 3 billion light years from the Earth. The most distant quasar has been named CFHQS J2329-0301 after the survey name and its coordinate position in the sky, which is in the constellation Pisces. Its red-shift of 6.43, corresponding to a distance of 13 billion light-years! Until the recent observation of huge gamma-ray bursts, quasars were the farthest known and brightest objects in the universe. The lower red-shift values are controversial. It may be these are Seyfert 2 galaxies rather than quasars. If Quasars are this close it would be contradictory to the current theories of how and when a Quasar forms.

When we look deep into space we are also looking deep into the past since we see the objects as they were when the light that took billions of years to get here was first emitted by the object. Because of the location of Quasars at cosmological distances it appears that they form when the host galaxy is still young. Since we see no Quasars in nearby galaxies it seems that they do not form in older galaxies and that they do not last past a galaxy’s middle age. There are recent observations of Quasars as close as z=0.06. These are controversial and are thought to be misidentified Seyfert 2 galaxies. If this is not the case and the categorizing of these objects as Quasars is correct, there may need to be significant modification to current models of Quasars.

Life

The life of a Quasar is probably not that long in cosmological terms. They begin to appear in galaxies that are relatively young but don’t last past middle age. We know it can’t take very long for a Quasar to form since the most distant we see appeared when the universe was only about 700 million years old. Since there are no (or very, very few) Quasars in the galaxies cosmologically near to use and the peak is at z=1.5 to z=3.5, they don’t appear to last for more than five billion or so years.

Other Information

New type of Quasar confirmed?

Type-2 quasars are new population of objects at cosmological redshift which are being revealed by X-ray satellites with new technology, such as ASCA. They are as luminous as ordinary quasars in hard X-ray wavelength but less luminous in optical, UV, and soft X-ray wavelength due to the large obscuration by the matter surrounding nuclei. Type-2 quasars are thought to be quasars whose axis of the anisotropic radiation is misdirected to our line-of-sight. Using type-2 quasars as probes we can learn much about quasar host objects, thanks to their relatively low contrast between nuclei and surrounding objects, and approach the relationship between quasar activity observed up to redshift of 5 and galaxy-formation phenomena.