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Quasars trip the light fantastic in the ancient Universe, beckoning the curious among us with their mesmerizing, mysterious, and brilliant dance among the distant stars. Quasars, or quasi-stellar radio sources, are the extraordinarily luminous active galactic nuclei (AGN) of galaxies far, far away–the accretion disks that surround supermassive black holes hiding in the dark hearts of probably every large galaxy in the Universe–including our own Milky Way. As the glowing gas in the accretion disk spirals downward into the waiting maw of the hungry black hole, an enormous amount of energy is released in the form of electromagnetic radiation–thus creating the bright quasar. In July 2018, a team of astronomers announced their discovery of a remote quasar displaying the most brilliant radio emission ever observed in the ancient Universe, resulting from a jet of extremely fast-moving material that the black hole has sent screaming out into space. These findings will enable astronomers to better probe the well-kept secrets of our Universe’s youth, during an important era of transition from what it was then to what it is now.
Dr. Eduardo Banados of the Carnegie Observatories (Pasadena, California) led the team that discovered the brilliant quasar, and this initial discovery was followed up by Dr. Emmanuel Momjian of the National Radio Astronomy Observatory (NRAO). The new observations enabled the astronomers to see, with unprecedented detail, the brilliant jet fleeing out of the quasar that formed within the first billion years of our 14 billion-year-old Universe’s existence. The NRAO is in Charlottesville, Virginia.
Quasars are comprised of supermassive black holes that are millions to billions of times more massive than our Sun. The newly discovered quasar, with the telephone book-sounding name of PSO J352.4034-15.3373, is one of a rare breed among its dark-hearted kind. These uncommon beasts inhabiting the celestial zoo do not merely swallow the matter they are in the process of consuming, but also shoot out a jet of plasma screeching out at speeds approaching that of light. This relativistic jet of material is extremely bright in the frequencies detected by radio telescopes. Even though quasars were first discovered over half a century ago as a result of their powerful radio emissions, astronomers now know that only approximately 10 percent of them are strong radio emitters.
Brilliant Beacons Inhabiting Spacetime
Quasars shoot brilliant streams of light into the Cosmos, and this fleeing light is as brilliant as a trillion stars. Indeed, quasars send forth their fabulous, ferocious light into intergalactic space from an area that is smaller than our Solar System. At the time of their discovery, these brilliant beacons were believed to be solitary stellar objects, situated in the dazzling hearts of ancient galaxies in the distant Universe. In astronomy, long ago is the same as far away. The more remote an object is in Space, the more ancient it is in Time. This is because of the expansion of the Universe–hence the term Spacetime. Time is the fourth dimension–it is impossible to locate an object in Space, without also locating it in Time. No known signal can travel faster than light in a vacuum, and the light that makes its incredibly long journey towards our own planet was emitted by remote objects inhabiting the Cosmos very long ago–and so we see them as they were when they first shot their dazzling light out into the space between galaxies.
Today, astrophysicists understand that supermassive black holes, inhabiting the hearts of galaxies, are the true “engines” that power quasars. In spite of their misleading name, black holes are not merely empty space. Indeed, they are really enormous amounts of matter squished into an extremely small region–and they do not come in only one size. Stellar mass black holes are born from the wreckage left behind by very massive stars that have perished in a supernova blast. Massive stars reach the end of their nuclear-fusing “lives” after having burned their necessary supply of fuel. The doomed massive star’s core collapses as its outer gaseous layers are shot violently into the space between stars–leaving only a black hole of stellar mass behind as its legacy to the Cosmos.
Supermassive black holes are a somewhat different beast from their smaller stellar-mass kin. This is primarily because of the stupendous mass of these gravitational supermassive monsters. The question that still needs to be answered is how such extremely massive entities came to exist in the first place. There is also evidence that intermediate mass black holes lurk in the Cosmos, and they sport hefty masses that exceed their stellar-mass “cousins”, but are nowhere near that of the supermassive beasts that lurk in the dark hearts of large galaxies.
Our own Milky Way Galaxy’s supermassive black hole is called Sagittarius A*–Sgr A*, for short (pronounced Saj-A-Star). Sgr A* is not as massive as many others of its kind, and is “only” millions–as opposed to billions–of solar-masses.
Quasars haunt the remote dark hearts of young, and extremely active, galaxies in the ancient Universe–and they hurl into space as much as a thousand times the energy produced by our entire Milky Way. This radiation is emitted across the entire electromagnetic spectrum almost uniformly, extending from the far-infrared to X-rays, displaying a peak in the ultraviolet-optical bands. Some quasars are powerful radio sources, as well as sources of gamma-rays.
It appears that the larger the host galaxy, the more massive its supermassive black hole. For this reason, it is frequently proposed that there must be some mechanism that ties the formation of a host galaxy to that of its resident supermassive heart of darkness–and vice versa. This theory carries important implications for various theories of galactic formation and evolution, and it is an ongoing avenue of research for astrophysicists.
Quasars were first identified during the 1950s as sources of radio-wave emission of unknown physical origin. When quasars were originally detected in photographic images from that early era they resembled dim star-like points of light. More recent high-resolution images of quasars, particularly those obtained from the Hubble Space Telescope (HST), have revealed that quasars are located in the cores of galaxies, and that some galaxies that host quasars are strongly interacting or merging galaxies. As with other types of AGN, the observed properties of a quasar depend on numerous factors including the mass of the resident supermassive black hole, the rate of the accretion of gas, the orientation of the surrounding accretion disk relative to the observer, the degree of veiling obscuration by gas and dust swirling within the host galaxy, and the presence or absence of a relativistic jet.
Quasar activity was more common in the very ancient Universe. Indeed, the peak era of quasar activity in the Universe occurred approximately 10 billion years ago.
The term “quasar” itself was coined by the Chinese-born U.S. astrophysicist Dr. Hong-Yee Chin in May 1964. Dr. Chin first used the term quasar in the journal Physics Today to describe certain mysterious astronomical objects.
Between 1917 and 1922, astronomers began to realize, based on the work conducted by Heber Curtis, Ernst Opik, and others, that there are objects in the Universe (nebulae) that are actually remote galaxies like our own Milky Way. However, when radio astronomy began to be used in the 1950s, astronomers spotted, among the galaxies, a small number of unusual objects that displayed properties that could not be explained.
The mysterious objects emitted copious amounts of radiation of numerous frequencies, but no source could be located optically–or (in some cases) only a point-like and very dim object could be seen, and these faint objects resembled remote stars. The spectral lines of these strange objects, which served to identify their constituent chemical elements, were also highly unusual and defied explanation. Some of them changed their luminosity very quickly in the optical range, and even more quickly in the X-ray range. This suggested to astronomers an upper limit on the size of these objects that was no larger than our own Solar System. This implies a high power density, and considerable debate occurred over what these mysterious, remote “star-like” objects could be. They were ultimately described as “quasi-stellar”–meaning “star-like” radio sources–or, alternatively, as “quasi-stellar objects” (QSOs), a designation that implied their unknown nature, and this eventually became shortened to “quasar”.
Most quasars cannot be observed with small telescopes. Although light and other radiation cannot escape the powerful gravitational grasp of a black hole’s event horizon, the energy churned out by a quasar is actually generated outside of the black hole. This energy is produced by gravitational stresses and immense friction within the material closest to the black hole, as it orbits and then spirals downward–never to return. The dazzling luminosity of quasars is caused by the accretion disks circling around and feeding the dark-hearted supermassive beast that awaits its dinner–and that can convert between 6% and 32% of the mass of an object into energy. Even though several dozen galaxies in our Milky Way’s general neighborhood–as well as our Milky Way itself–do not sport an active center and do not display any activity similar to a quasar, they nevertheless host a quiet supermassive black hole in their hidden dark hearts. For this reason, it is now thought that all large galaxies host a supermassive black hole in their nuclei (center). However, only a small percentage become active and power radiation in this way. It is the activity of these ancient supermassive black holes that are observed as brilliant quasars.
The material that tumbles down fatally into the waiting maw of the supermassive black hole, is unlikely to fall directly in. Instead, it will have some angular momentum around the gravitational beast that will cause the matter to gather together in an accretion disk. Quasars may ignite (or be re-ignited) when a duo of normal galaxies collide and then merge together. When this type of collision occurs, the supermassive black hole can then feast on a fresh new source of matter. Indeed, it has been suggested that a dazzling quasar may be born when the nearby large spiral Andromeda Galaxy collides and merges with our own Milky Way in 3 to 5 billion years–thus creating the great Milkomeda Galaxy that will host a supermassive black hole in its secretive heart. The Milkomeda Galaxy’s supermassive black hole will sport the impressive mass of both of the colliding duo’s supermassive black holes combined.
More than 200,000 quasars have been discovered so far, most detected by the Sloan Digital Sky Survey (SDSS). These quasars have been determined to be located between 600 million and 28.85 billion light-years from Earth (in terms of comoving distance). Because of the vast distances to the most remote quasars, and the finite speed of light, these distant dazzling objects and their ambient space are observed as they were in the ancient Cosmos. The SDSS is a multi-spectral imaging spectroscopic redsift survey using the dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico in the U.S.
Even though quasars are faint when observed from our own planet, they are visible from great distances. This is because they are the most luminous objects known in the Universe.
One particularly nagging question bewilders and bewitches astrophysicists–why do the galaxies in our Milky Way’s general neighborhood all seem to host dormant supermassive beasts in their hidden hearts? Our own Sgr A* is just such a slumbering old black hole. The brilliance of its ancient youth has long since passed, and Sgr A* is only from time to time awakened from its nap when a tempting shredded star or cloud of doomed gas travels too close to where it slumbers in the gentle peace of its “golden years”. When this happens, the beast awakens, and devours the infalling banquet with the greed of its long gone flaming youth. For one brief shining moment, the elderly gravitational beast remembers what it once was, and glares with the same fiery fury that it did very long ago when both it and the Universe itself were young.
Indeed, many astrophysicists think that Sgr A* was once a quasar, that sparkled like a brilliant beacon in the ancient Universe.
A Tattle-Tale Relativistic Jet
The light emitted by the newly discovered quasar, PSO J352.4034-15.3373, has been traveling from its place of origin for almost 13 billion years, and it is only now finally reaching the prying eyes of astronomers on Earth. P352-15 (as it is known, for short) is the first quasar to be observed that displays strong evidence of radio jets within the first billion years of the almost 14 billion-year-old Universe’s existence.
“There is a dearth of known strong radio emitters from the Universe’s youth and this is the brightest radio quasar at that epoch by an order of magnitude,” Dr. Banados explained in a July 9, 2018 Carnegie Science Press Release.
“This is the most-detailed image yet of such a bright galaxy at this great distance,” Dr. Momjian added in the same Press Release.
The Big Bang started the Universe as a searing-hot cauldron filled with extremely energetic particles that were rapidly expanding. As the Universe expanded, it cooled off and coalesced into a neutral hydrogen gas, which left the Universe swathed in darkness. During this Cosmic Dark Ages, there were no luminous sources, until the relentless force of gravity condensed matter that ultimately set fire to the first generation of brilliant baby stars and their host galaxies. Approximately 800 million years after the Big Bang, the energy released by the first galaxies caused the neutral hydrogen that was spread throughout the Universe to get excited. In other words, the neutral hydrogen lost an electron, or became ionized–a state that the gas has remained in since that ancient era.
It is rare to find radio jet-emitting quasars, such as P352-15, from the primordial period just after the Universe’s lights switched back on.
Dr. Banados commented to the press on July 9, 2018 that “The jet from this quasar could serve as an important calibration tool to help future projects penetrate the (cosmic) Dark Ages and perhaps reveal how the earliest galaxies came into being.”
The new findings are published in two papers appearing in the July 9, 2018 issue of The Astrophysical Journal.