Quasars: Beacons of the Distant Cosmos

An Universe Illuminated by its Most Violent Cores

Discovery and Power of Quasars

Quasars (for "quasi-stellar radio source") are among the brightest and most energetic objects in the known Universe. Discovered in the early 1960s by Maarten Schmidt (1929-2022), they emit radiation capable of rivaling the light of hundreds of entire galaxies. Their power comes from accretion onto a supermassive black hole located at the center of an active galaxy.

Physical Nature of a Quasar

A quasar is neither a simple black hole nor a galaxy. It represents a state of activity of the galactic nucleus, not a separate type of object. The quasar refers to the extreme activity phenomenon observed at the center of certain galaxies, not the material object itself.

Physical Conclusion

The black hole is the energy engine, the quasar the observable phenomenon produced by this engine, and the host galaxy the gravitational framework containing the entire system.

Luminosity and Energy

The total luminosity of a quasar can reach \( L \approx 10^{40}\ \mathrm{W} \), about \(10^{13}\) times that of the Sun. Such energy can only originate from an efficient gravitational mechanism. Mass-energy conversion follows the relativistic equation \(\,E = \eta m c^2\,\), where \(\eta\) is the efficiency, typically between 0.1 and 0.4 in a relativistic accretion disk.

Note:
An active galaxy has an extremely bright nucleus powered by accretion of gas onto a supermassive black hole. This energy, around \(10^{39}\) to \(10^{41}\ \mathrm{W}\), described by \(\,E = \eta m c^2\,\) with \(\eta \approx 0.1\), makes AGN the most powerful sources in the observable Universe.

Extreme Distances and High Redshifts

Quasars are at cosmological distances, sometimes over 12 billion light-years away. Their spectra show high redshift (\(z > 6\)), indicating emission when the Universe was only a fraction of its current age.

These measurements allow astrophysicists to study the epoch of cosmic reionization, the transition of the Universe from a neutral to an ionized state. Quasars, illuminating the intergalactic medium, serve as temporal beacons of cosmic structure formation.

Redshift and Quasar Distances

Accretion Physics and Relativistic Jets

Around the central black hole, matter forms a rapidly rotating disk. Internal friction converts gravitational potential energy into electromagnetic radiation.

Strong magnetic fields channel part of the energy into relativistic jets, capable of propelling particles at speeds near \(c\), the speed of light. These jets, observed notably by the VLBA, are a major signature of Active Galactic Nuclei (AGN).

Quasars and Galactic Evolution

Observations suggest most massive galaxies host a supermassive black hole at their center. Quasars may represent a transitional phase in this galactic evolution: when a gas inflow triggers rapid accretion, the nucleus becomes dazzling. Once the matter reservoir is depleted, the galaxy stabilizes and the quasar shuts down.

The observed correlation between black hole mass (\(M_{\mathrm{BH}}\)) and galactic bulge mass (\(M_{\mathrm{bulge}}\)) suggests coevolution governed by energetic feedback: \( M_{\mathrm{BH}} \propto M_{\mathrm{bulge}}^{1.1} \)

Comparative Table: Quasars and Other Active Nuclei

Source: NASA ADS Astrophysics Data System, European Southern Observatory (ESO).