Quasars: The brightest objects in the universe

Shining so brightly these astronomical objects outshine the galaxies they reside in, which makes them visible from a distance of  billions of light years. Astronomers believe that quasars are the most distant and luminous objects in the universe. They are found in the centre of galaxies often powered by a supermassive black hole. However, these black holes do not emit visible light or radio light, the light we see from quasars comes from a disk of gas spiraling around the black hole. Quasars also emit jets, collimated beams of matter ejected from an astronomical object. Though quasars have a high luminosity they cannot be seen by the naked eye, this is because the nearest quasars are a billion parsecs away.

Source: NASA/ESA

Discovery of Quasars

The word “quasar” and the acronym “QSO” are short for “quasi-stellar radio source” and “Quasi-stellar object” respectively, due to their ‘star-like’ appearance. The term ‘quasi’ derives from how they were originally discovered in the earliest radio surveys of the skies in the 1950s. Their optical spectrum was an intriguing mystery because some radio sources claimed that the  object appeared to be unusually blue stars. 

Source: AURA/STScI/NASA/JPL

The photographs taken of these spectra showed emission lines at wavelengths that were at odds with all the celestial objects known to mankind. The mystery was resolved by the Dutch American astronomer Maarten Schmidt, who in 1963 recognized that the pattern of these emission lines in 3C 273, the brightest known quasar, could be understood as coming from hydrogen atoms that had a redshift, had their emission lines shifted toward longer and redder wavelengths by the expansion of the universe,of 0.158. Due to the redshift of this magnitude, 3C 273 was placed nearly two billion light years away using Hubble's law.

Source: NASA/STScI/ESA

The quasar ,3C 273, was about 100 times more luminous than the brightest individual galaxies in the clusters found near it, and nothing so bright had been seen so far away. Another shocking discovery was made when the levels of the brightness of this quasar started varying on timescales as short as a few days, meaning that the total size of the quasar cannot be more than a few light-days across. Since the quasar is so luminous and compact, its radiation pressure has to be huge, suggesting that the only way to keep the quasar from exploding is because of its own radiation is that it is atleast a few solar masses.

Family Tree of Quasars

Quasars are part of a class of objects known as active galactic nuclei (AGN). More examples of the same include Seyfert galaxies and blazarhat the three types of AGNs are the same objects, but they all have different perspectives.Many astronomers believe that the three types of AGNs are the same objects, but with different perspectives. While the jets of quasars seem to stream at an angle generally in the direction of Earth, blazars may point their jets directly toward the planet. Although no jets are seen in Seyfert galaxies, scientists think this may be because we view them from the side, so all of the emission is pointed away from us and thus goes undetected.

Source: NASA/ESA

Finding Quasars

The first ever quasar was discovered by radio sources  yet it was not considered the most efficient method to detect quasars. The quasars could be found more efficiently by looking for objects bluer than normal stars. This can be done with relatively high efficiency by photographing large areas of the sky through two or three different-coloured filters. The photographs are then compared to locate the unusually blue objects, whose nature is verified through subsequent spectroscopy. This remains the primary technique for finding quasars, although it has evolved over the years with the replacement of film by electronic charge-coupled devices (CCDs), the extension of the surveys to longer wavelengths in the infrared, and the addition of multiple filters that, in various combinations, are effective at isolating quasars at different redshifts. Many quasars have also been discovered through other techniques, including searches for starlike sources whose brightness varies irregularly and X-ray surveys from space; indeed, a high level of X-ray emission is regarded by astronomers as a sure indicator of an accreting black-hole system.

Evolution of Quasars

The number density of quasars increases dramatically with redshift, which means through Hubble’s law to more quasars at larger distances. When quasars are observed at great distances, they are observed as they were in the distant past, because of the finite speed of light. Thus, the increasing density of quasars with distance means that they were more common in the past than they are now. The big bang occurred approximately 13.5 billion years ago and the quasars came into existence around three billion years after the big bang. At earlier ages, the number density of quasars decreased sharply, corresponding to an era when the quasar population was still building up. The most distant, and thus earliest, quasars known were formed less than a billion years after the big bang.

Source:AURA/STScI/NASA/JPL

Individual quasars appear possibly triggered by a merger with another galaxy as the black hole in their centre begins to accrete gas at a high rate. This gives an estimate that  quasar activity is episodic, with individual episodes lasting around a million years and the total quasar lifetime lasting around 10 million years. At some point, quasar activity ceases completely, leaving behind the dormant massive black holes found in most massive galaxies.