A the most distant known quasars). The brightest quasars

A quasar or quasi-stellar object (QSO) is an active galactic nucleus (AGN) having extremelyhigh luminosities (L ? 10 12 ? 10 15 L 1 ). A quasar consists of a supermassive black hole atthe centers of galaxies. These black holes cause an “accretion” disc around the quasars;this accreted matter is diverted at both ends of the black hole, like jets of matter. It isthis matter that we observe here on Earth. An artist impression of what a typical quasarmay look like is shown in Fig. 1. Quasars emit energy across the broad electromagneticspectrum and can be observed at radio, infrared, visible, ultraviolet, and X-ray wavelengths.The most powerful quasars have luminosities exceeding 10 48 erg s ?1 , thousands of timesgreater than the luminosity of Milky Way galaxy.The term “quasar” is an abbreviation for “quasi-stellar radio source”, because firstquasars were originally discovered as to be sources with radio emission in the 1950s.Looking away from the plane of our Milky Way galaxy, most of the radio sources werediscovered with normal looking galaxies. However, few of these radio sources coincidedwith objects that appeared to be unusually blue stars. This star-like feature is also clearlyvisible in photographic images taken with various telescopes/spectrographs at visiblewavelengths. Apart from having an appearance like a star, some of these objects inimages looked to be embedded in faint and fuzzy halos. The term “quasar” was coinedby Chinese-born U.S. astrophysicist Hong-Yee Chiu in May 1964, in Physics Today, todescribe these star-like objects. Later it was found that not all quasars have strong radioemission; in fact only about 10% are “radio-loud”. Hence the name quasi-stellar objector ‘QSO’ is used (in addition to “quasar”) to refer to these objects, including both the’radio-loud’ and the ‘radio-quiet’ classes. Most astronomers have regarded QSOs to be atthe high-luminosity end of an even larger population of “active galactic nuclei” or AGNs.The lower luminosity AGNs are known as “Seyfert galaxies”. High-resolution images ofquasars from the Hubble Space Telescope (HST) or Very Large Telescope (VLT) havedemonstrated that quasars occur in the centers of galaxies, and that some quasar hostgalaxies are strongly interacting or merging galaxies.The incredibly high luminosities make quasars visible out to over a very broad rangeof distances (corresponding to redshifts ranging from z < 0.1 for the nearest quasars toz > 7 for the most distant known quasars). The brightest quasars can outshine all of thestars in the galaxies in which they reside, which makes them visible even at distances ofbillions of light-years. As of 2017, the most distant known quasar is ULAS J1342+0928 atredshi z = 7.54 (or 13.1 billion light years away from the Earth). Light observed from thisquasar was emitted when the Universe was only 690 million years old. The supermassiveblack hole in this quasar is the most distant black hole identified to date, and is estimatedto have a mass that is 800 million times the mass of our SunMaarten Schmidt, a Caltech astronomer working at Mt. Palomar Observatory (see Fig. 2),was credited in 1963 with the discovery of quasars– the most distant and intrinsicallyextremely luminous object observed till date. Astronomers typically investigate objectsby studying their spectra as it presents a new mystery. In 1963, Maarten Schmidt wasstudying the radio source 3C 273 (see the left-hand side of Fig. 2). While analysing thespectrum of this radio source, he found it to be puzzling as he was unable to figure outwhat elements produced the bight spectral lines. Then he realized that the unfamiliar lines were simply the bright emission lines from hydrogen gas that had a redshift (i.e.,the emission lines were shifted toward longer, redder wavelengths by the expansion of theuniverse). It was observed that the wavelength of each line was 1.158 times longer thanthe wavelength measured in the laboratory, where the source is at rest with respect to theobserver. Thus the quasar 3C 273 is at a redshift of z = 0.158 (as shown in the right-handside of Fig. 2), which according to Hubble’s law of expansion puts it a distance of slightlymore than two billion light-years. However, bright clusters of galaxies had been identifiedat such similar distances, but 3C 273 is about 100 times more luminous than the brightestindividual galaxies in those clusters, and nothing so bright had been discovered so faraway.The continuing observations of quasars revealed that their luminosities can varysignificantly on timescales as short as a few days, meaning that the total size of thequasar cannot be more than a few light-days across. Since the quasar is very compact andextremely luminous, the radiation pressure inside the quasar must indeed be huge. Hence,