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Formation and Evolution of Quasars

What Feeds The Monster?

Quasars or QSOs were first discovered in 1963. Initially they appeared as mysterious objects. They were stellar like (hence the name Quasi- Stellar-Object) objects which had a strange emission line spectrum. An example is shown below:

The basic problem was that the emission lines were at wavelengths which were unfamiliar to astronomers. Either these were new atomic processes at work, or something very odd was going on.
After a few of these objects were discovered it was soon realized that the emission lines were from normal atomic processes (i.e. electronic transisitions in Hydrogen and Oxygen) but that the redshift of the "Quasi-stellar" objects was enormous.
Implications of the large redshifts:

Redshifts imply that the QSO distance is 5--10 billion light years from the earth. This is an appreciable fraction of the radius of the observable Universe.

The implied intrinsic energy output of these QSOs is enormous. Approximately 100 times the energy output of the entire Milky Way Galaxy

Since most QSOS are at very high redshift (long look back times) - this means that whatever they are they were preferentially formed when the Universe was young and by now they are dead

Further observations of QSOs showed the following set of characteristics as well:

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!

We now have a fairly major problem: --> What physical process can output so much energy in such a small volume?
Possible Energy Sources:

Supermassive Star: --> Very short lifetime
Super - Supernovae Explosion: Even Shorter Lifetime
White Holes (Lagging cores): --< mathematical and not physical
New Physics: --> very exciting and not completely ruled out
What about E=mc²
Energy Requirements:

Observed luminosity is about 100 times that of the Milky Way Galaxy. The luminosity of the Milky way is 10¹¹ L_sun so a QSO is

10² x 10¹¹ = 10¹³L_sun
1 L_sun = 4 x 10³³ ergs per second
10¹³L_sun x 4 x 10³³ ergs per second = 4 x 10⁴⁶ ergs per second
For a 1 M_sun star, E=mc² gives the following energy:

(2 x 10³³ grams) x (3 x 10¹⁰ cm/sec)²
which is then

(2 x 10³³ grams) x (9 x 10²⁰) (cm/sec)² = 1.8 x 10⁵⁴ ergs
The requirement is 4 x 10⁴⁶ ergs per second and since there are 3 x 10⁷ seconds in a year this is then 1.2 x 10⁵³ ergs per year.
Hence if we could convert 1 solar mass per year into energy with 10% efficiency we could reach the observed luminosities of QSOs.

What physical mechanism can produce this conversion?
There is only one possibility:
Gravitational Infall of matter onto a very large black hole. In this case the mass of the black hole needs to be about 10⁸ M_sun. A black hole this large has a radius of about 10 Astronomical Units (or about the distance between the Sun and Saturn).
A region this small is consistent with the short time scale variablity of emission that we measure.
So, are QSOs then naked -- that is, do we have these 10⁸ M_sun black holes roaming around the Universe looking for stars to eat? What is the mechanism that feeds these objects? If they are not fed matter, they do not produce luminosity on their own.
In the mid-80's it was determined from CCD imaging of some low redshift QSOs that the QSOs were located in the centers of galaxies and hence are just a more luminous form of Active Galactic Nuclei like Seyfert Galaxies . So if QSOs are hosted by galaxies then the only remaining challenge is to feed them.
Since QSOs are far more common when the Universe was young then the feeding mechanism must be fostered by conditions which were present then compared to now.
Many nearby QSOs show evidence of interacting with another galaxy. Galaxy-galaxy interactions are a good way of funnelling material down to the nucleus of a galaxy and if a monster is there, voila a QSO is produced. In general it is the transfer of gas onto a galactic nucleus that hosts a "monster" which activates the QSO. Interactions between gas-rich galaxies were far more common in the early Universe than now. This explains why most QSOs were borne a long time ago and have died out by now.
A lot of times the nuclear region will be very dusty and most of the emission produced comes out in the infrared. Very infrared luminuous objects known as Ultraluminous IRAS galaxies have been detected and are thought to be like QSOs. All of thse Ultraluminous IRAS galaxies are merging galaxies where gas is being forced down into the nucleus.
If this sceneraio is correct, then there should be nearby examples of dead QSOS --> that is galaxies with big massive black holes in them and the QSOs mechanism is no longer active. At the moment, only this galaxy (M87) seems to have one of these dead monsters in it.
Final note: the proliferation of QSOs in the early Universe and their associated intense UV radiation becomes a significant source of ionizing radiation which could help to delay the formation of galaxies until later times.
More about Quasars:

Quasars and the Early Universe

A small Quasar in the Milky Way Galaxy? ___________________________________________________________________
The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu