Quasar 17
Nadal Braymiller
A quasar (/ˈkweɪzɑːr/ KWAY-zar) is an extremely luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by a supermassive black hole with a mass ranging from millions to tens of billions of solar masses, surrounded by a gaseous accretion disc. Gas in the disc falling towards the black hole heats up and releases energy in the form of electromagnetic radiation. The radiant energy of quasars is enormous; the most powerful quasars have luminosities thousands of times greater than that of a galaxy such as the Milky Way.[2][3] Quasars are usually categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.[4]
Quasar discovery surveys have shown that quasar activity was more common in the distant past; the peak epoch was approximately 10 billion years ago.[15] Concentrations of multiple quasars are known as large quasar groups and may constitute some of the largest known structures in the universe if the observed groups are good tracers of mass distribution.
So far, the clumsily long name "quasi-stellar radio sources" is used to describe these objects. Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. 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 pik 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.
The objects emitted large amounts of radiation of many frequencies, but no source could be located optically, or in some cases only a faint and point-like object somewhat like a distant star. The spectral lines of these objects, which identify the chemical elements of which the object is composed, were also extremely strange and defied explanation. Some of them changed their luminosity very rapidly in the optical range and even more rapidly in the X-ray range, suggesting an upper limit on their size, perhaps no larger than the Solar System.[17] This implies an extremely high power density.[18] Considerable discussion took place over what these objects might be. They were described as "quasi-stellar [meaning: star-like] radio sources", or "quasi-stellar objects" (QSOs), a name which reflected their unknown nature, and this became shortened to "quasar".
The first quasars (3C 48 and 3C 273) were discovered in the late 1950s, as radio sources in all-sky radio surveys.[19][20][21][22] They were first noted as radio sources with no corresponding visible object. Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size.[23] By 1960, hundreds of these objects had been recorded and published in the Third Cambridge Catalogue while astronomers scanned the skies for their optical counterparts. In 1963, a definite identification of the radio source 3C 48 with an optical object was published by Allan Sandage and Thomas A. Matthews. 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. The anomalous spectrum defied interpretation.
British-Australian astronomer John Bolton made many early observations of quasars, including a breakthrough in 1962. Another radio source, 3C 273, was predicted to undergo five occultations by the Moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to find a visible counterpart to the radio source and obtain an optical spectrum using the 200-inch (5.1 m) Hale Telescope on Mount Palomar. This spectrum revealed the same strange emission lines. Schmidt was able to demonstrate that these were likely to be the ordinary spectral lines of hydrogen redshifted by 15.8%, at the time, a high redshift (with only a handful of much fainter galaxies known with higher redshift). If this was due to the physical motion of the "star", then 3C 273 was receding at an enormous velocity, around 47000 km/s, far beyond the speed of any known star and defying any obvious explanation.[24] Nor would an extreme velocity help to explain 3C 273's huge radio emissions. If the redshift was cosmological (now known to be correct), the large distance implied that 3C 273 was far more luminous than any galaxy, but much more compact. 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.
Although it raised many questions, Schmidt's discovery quickly revolutionized quasar observation. The strange spectrum of 3C 48 was quickly identified by Schmidt, Greenstein and Oke as hydrogen and magnesium redshifted by 37%. Shortly afterwards, two more quasar spectra in 1964 and five more in 1965 were also confirmed as ordinary light that had been redshifted to an extreme degree.[25] While the observations and redshifts themselves were not doubted, their correct interpretation was heavily debated, and Bolton's suggestion that the radiation detected from quasars were ordinary spectral lines from distant highly redshifted sources with extreme velocity was not widely accepted at the time.
An extreme redshift could imply great distance and velocity but could also be due to extreme mass or perhaps some other unknown laws of nature. Extreme velocity and distance would also imply immense power output, which lacked explanation. 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. But if they were small and far away in space, their power output would have to be immense and difficult to explain. Equally, if they were very small and much closer to this galaxy, it would be easy to explain their apparent power output, but less easy to explain their redshifts and lack of detectable movement against the background of the universe.
Schmidt noted that redshift is also associated with the expansion of the universe, as codified in Hubble's law. If the measured redshift was due to expansion, then this would support an interpretation of very distant objects with extraordinarily high luminosity and power output, far beyond any object seen to date. This extreme luminosity would also explain the large radio signal. Schmidt concluded that 3C 273 could either be an individual star around 10 km wide within (or near to) this galaxy, or a distant active galactic nucleus. He stated that a distant and extremely powerful object seemed more likely to be correct.[26]
Schmidt's explanation for the high redshift was not widely accepted at the time. A major concern was the enormous amount of energy these objects would have to be radiating, if they were distant. In the 1960s no commonly accepted mechanism could account for this. The currently accepted explanation, that it is due to matter in an accretion disc falling into a supermassive black hole, was only suggested in 1964 by Edwin E. Salpeter and Yakov Zeldovich,[27] and even then it was rejected by many astronomers, as at this time the existence of black holes at all was widely seen as theoretical.
Various explanations were proposed during the 1960s and 1970s, each with their own problems. 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. This would require a massive object, which would also explain the high luminosities. However, a star of sufficient mass to produce the measured redshift would be unstable and in excess of the Hayashi limit.[28] 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.[29] There were also serious concerns regarding the idea of cosmologically distant quasars. One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including nuclear fusion. There were suggestions that quasars were made of some hitherto unknown stable form of antimatter in similarly unknown types of region of space, and that this might account for their brightness.[30] Others speculated that quasars were a white hole end of a wormhole,[31][32] or a chain reaction of numerous supernovae.[33]
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.[34] This included crucial evidence from optical and X-ray viewing of quasar host galaxies, finding of "intervening" absorption lines, which explained various spectral anomalies, observations from gravitational lensing, Gunn's 1971 finding that galaxies containing quasars showed the same redshift as the quasars,[35] and Kristian's 1973 finding that the "fuzzy" surrounding of many quasars was consistent with a less luminous host galaxy.[36]
c80f0f1006