Quasar

For convenience, the abbreviated form "quasar" will be used throughout this paper.Between 1917 and 1922, it became clear from work by Heber Doust Curtis, Ernst Öpik and others that some objects ("nebulae") seen by astronomers were in fact distant galaxies like the Milky Way.

But when radio astronomy began in the 1950s, astronomers detected, among the galaxies, a small number of anomalous objects with properties that defied explanation.

Astronomers had detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum, which contained many unknown broad emission lines.

Also, 3C 273 was bright enough to detect on archival photographs dating back to the 1900s; it was found to be variable on yearly timescales, implying that a substantial fraction of the light was emitted from a region less than 1 light-year in size, tiny compared to a galaxy.

The small sizes were confirmed by interferometry and by observing the speed with which the quasar as a whole varied in output, and by their inability to be seen in even the most powerful visible-light telescopes as anything more than faint starlike points of light.

It was suggested that quasars were nearby objects, and that their redshift was not due to the expansion of space but rather to light escaping a deep gravitational well.

[23] Quasars also show forbidden spectral emission lines, previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well.

[28] Eventually, starting from about the 1970s, many lines of evidence (including the first X-ray space observatories, knowledge of black holes and modern models of cosmology) gradually demonstrated that the quasar redshifts are genuine and due to the expansion of space, that quasars are in fact as powerful and as distant as Schmidt and some other astronomers had suggested, and that their energy source is matter from an accretion disc falling onto a supermassive black hole.

The accretion-disc energy-production mechanism was finally modeled in the 1970s, and black holes were also directly detected (including evidence showing that supermassive black holes could be found at the centers of this and many other galaxies), which resolved the concern that quasars were too luminous to be a result of very distant objects or that a suitable mechanism could not be confirmed to exist in nature.

[34] In 1979, the gravitational lens effect predicted by Albert Einstein's general theory of relativity was confirmed observationally for the first time with images of the double quasar 0957+561.

[36] In March 2021, a collaboration of scientists, related to the Event Horizon Telescope, presented, for the first time, a polarized-based image of a black hole, specifically the black hole at the center of Messier 87, an elliptical galaxy approximately 55 million light-years away in the constellation Virgo, revealing the forces giving rise to quasars.

[37] It is now known that quasars are distant but extremely luminous objects, so any light that reaches the Earth is redshifted due to the expansion of the universe.

[38] Quasars inhabit the centers of active galaxies and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200–400 billion stars.

This radiation is emitted across the electromagnetic spectrum almost uniformly, from X-rays to the far infrared with a peak in the ultraviolet optical bands, with some quasars also being strong sources of radio emission and of gamma-rays.

Quasars are believed—and in many cases confirmed—to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, as suggested in 1964 by Edwin Salpeter and Yakov Zeldovich.

[41] This also explains why quasars were more common in the early universe, as this energy production ends when the supermassive black hole consumes all of the gas and dust near it.

Quasars may also be ignited or re-ignited when normal galaxies merge and the black hole is infused with a fresh source of matter.

Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as they existed in the very early universe.

The Doppler shifts of stars near the cores of galaxies indicate that they are revolving around tremendous masses with very steep gravity gradients, suggesting black holes.

One idea is that jets, radiation and winds created by the quasars shut down the formation of new stars in the host galaxy, a process called "feedback".

To create a luminosity of 1040 watts (the typical brightness of a quasar), a supermassive black hole would have to consume the material equivalent of 10 solar masses per year.

Quasars can be detected over the entire observable electromagnetic spectrum, including radio, infrared, visible light, ultraviolet, X-ray and even gamma rays.

A minority of quasars show strong radio emission, which is generated by jets of matter moving close to the speed of light.

When viewed downward, these appear as blazars and often have regions that seem to move away from the center faster than the speed of light (superluminal expansion).

Radio-loud quasars can also produce X-rays and gamma rays by inverse Compton scattering of lower-energy photons by the radio-emitting electrons in the jet.

The oldest known quasars (z = 6)[needs update] display a Gunn–Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas.

More recent quasars show no absorption region, but rather their spectra contain a spiky area known as the Lyman-alpha forest; this indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.

Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky.

[67] The International Celestial Reference System (ICRS) is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky.

[71] In 2013, the second true triplet of quasars, QQQ J1519+0627, was found with a redshift z = 1.51, the whole system fitting within a physical separation of 25 kpc (about 80,000 light-years).