What are Quasars?
The word quasar stands for quasi-stellar radio source. Quasars got that name because they looked star-like when astronomers first began to notice them in the laste 1950s and early 60s. But quasars are not stars. Scientists now know they are young galaxies, located at vast distances from us, with their numbers increasing towards the edge of the visible universe. How can they be so far away and yet still visible?The answer is that quasars are extremely bright,up to 1000 times brighter than our Milky way galaxy. We know, therefore that they are highly active, emitting staggering amounts of radiation across the entire electromagnetic spectrum.
More than a million quasars have been found,with the nearest known being about 600 million light-years form Earth. he record for the most distant known quasar continues to change. In 2017,quasar ULAS J1342+0928 was detected at redshift z=7.54. Light observed form this 800 million solar-mass quasar was emitted when the universe was only 690 million years old.In 2020, quasar Pōniuāʻena or J100758.264+211529.207 or J1007+2115 was detected from a time only 700 million years after the big bang, and with an estimated mass of 1.5 billion times the mass of the Sun. In early 2021, the quasar OSO J0313-1806, with a 1.6 billion-solar-mass black hole, was reported at z=7.64, 670 million years after the Big Bang. Quasar discovery surveys have shown that quasar activity was more common in the distant past;the peak was approximately 10 billion years ago. Concentrations of multiple,gravitationally attracted quasars are known as large quasar groups and constitute some of the larget known structures in the universe.
Supermassive black holes power the bright nuclei of young galaxies in
the early universe — quasars. But how do these black holes gain
supermassive status in less than a billion years?
Supermassive Centers
Quasars are truly superlative objects — they are the brightest objects we know of, and some astronomers claim that they’re also the most interesting! Quasars are so luminous that we can observe them at outrageous distances — the most distant quasar discovered shines from roughly 13 billion light-years away. This vast distance means that we see quasars as they were when the universe was less than a billion years old, on the edge of the epoch of reionization(In the fields of Big Bang theory and cosmology, reionization is the process that caused electrically neutral atoms in the universe to reionize after the lapse of the "dark ages".), when the first stars and galaxies suffused the universe with photons and put an end to the cosmic dark ages. These ultra-bright objects are thought to be the nuclei of young galaxies, powered by the accretion of material onto a central supermassive black hole. The presence of supermassive black holes so early in the universe’s history poses a challenge for theorists. How, exactly, does a black hole amass so much material in just a few hundred million years?Sensitive Spectroscopy
The size of a supermassive black hole in the early universe is
determined by the masses of the smaller black hole “seeds” from which it
forms as well as the rate at which it accretes gas from its
surroundings. In order to determine the masses and accretion rates of
young supermassive black holes, a team led by Jinyi Yang (Steward
Observatory, University of Arizona) analyzed infrared spectra of 37
quasars with redshifts between 6.3 and 7.64 — roughly 700 to 900 million
years after the Big Bang. Yang and collaborators calculated the masses of the black holes in
their sample to be in the range of 300 million to 3.6 billion solar
masses and found that they accrete at 0.26 to 2.3 times the Eddington
limit — the theorized point at which the outward push of radiation
generated by accretion is so strong that it balances the inward pull of
gravity.
Gauging Growth
The authors estimate that the black holes in their sample must have arisen from black hole seeds no smaller than 1,000 to 10,000 solar masses — but it’s not clear what process could generate these seeds. The collapse of the first generation of stars is one possibility, but these massive stars likely only formed black holes of a few hundred solar masses. Another option is the collapse of gas clouds directly into black holes without first forming stars, but this process is thought to be extremely rare. Even if the seeds are small, rapid accretion could still bulk these early-universe black holes up to the masses we observe. However, in order to reach billions of solar masses, early-universe black holes would need nearly a billion years of sustained accretion at a rate far exceeding the Eddington limit — anywhere from a few to a few thousand times this limit, depending on the seed mass — and most of the quasars in this study are accereting too slowly. The jury is still out on how supermassive black holes in the early universe gain their impressive size — perhaps in addition to being the most luminous and most interesting objects in the universe, quasars are also the most mysterious!.jpg)
Development of physical understanding (1960s)
Redshits at extreme can indicate great distance and velocity, but they can also be caused by extreme mass or some other unknown law of nature. In addition to extreme velocity and distance, extreme power output would also be inconceivable without an explanation. By interferometry and by observing how the quasar's output varied with time, as well as by their inability to be seen with even the most powerful visible-light telescopes as anything more than faint stars, the small size of the quasars was confirmed. The power output of such a system would have to be enormous and difficult to explain if it were small and far from the earth. It would also be easier to explain their apparent power output if they were smaller and closer to this galaxy, but less easy to explain their redshifts and lack of movement against the background of the universe if they were much smaller.
It was also noted by Schmidt that redshift is also linked with the expansion of the universe as shown by Hubble's law. An explanation for the measured redshift could be very distant objects with extraordinarily high luminosity and power output, much greater than anything observed so far. This extreme brightness could also explain the huge radio signals. Schmidt concluded that 3C 273 could be a single star about 10 kilometers across in (or near) the galaxy, or it could be a distant active galactic nucleus. He said it was more likely that there was a distant and extremely powerful objects.
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Schmidt's explanation for the high redshift was not widely accepted at the time. One major concern is that if these objects are very far away, they will have to radiate a lot of energy. In the 1960s, there was no accepted mechanism to explain this. The currently accepted explanation is that the material in the accretion disk fell into the supermassive black hole. This explanation was not proposed until 1964 by Edwin E. Salpeter and Yakov Zeldovich. At that time, it was rejected by many astronomers, as was the existence of black holes, they were only considered theoritical. Various explanations were proposed in the 1960s and 1970s, but each had its own problems. It has been suggested that quasars are nearby objects whose redshift is due not to the expansion of space but to light escaping from deep gravitational sources. This would require a huge object, which would also explain the high luminosity. However, a star massive enough to produce the measured redshift would be unstable and exceed the Hayashi limit.
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