The Mystery Of The Most Distant Dark Heart
By Judith E Braffman-Miller
When astronomers look deeper and deeper into Space, they are staring further and further back in time. This is because the more remote a shining object is in Space, the longer it has taken its fleeing beams of light to reach us. There is no known signal that can fly faster than light in a vacuum, and the light that travels from distant objects in the Universe cannot flow faster than this universal speed limit will allow. Supermassive black holes. that possess extraordinary masses of millions to billions of times that of our Sun, are thought to haunt the hidden hearts of probably every large galaxy in the Universe–including our own starlit pin-wheel in Space and Time, our large spiral Milky Way Galaxy. In December 2017, a team of astronomers led by the Carnegie Institution’s Dr. Eduardo Banados, used Carnegie’s Magellan telescopes to discover the most-distant dark-hearted supermassive beast ever observed. This enormous black hole resides in sinister secret within a brilliantly luminous quasar, and its traveling light reaches us from the very ancient time when the Universe was only 5% of its current age–a mere 690 million years after the Big Bang. A paper describing this new discovery is published in the journal Nature.
Quasars are glaring, brilliant objects, composed of enormous hungry black holes that are in the process of accreting matter. This unfortunate material is paying the ultimate price for wandering too close to where the supermassive beast lurks secretively in the centers of massive galaxies–waiting for its dinner. This recently discovered supermassive black hole sports a mass that is 800 million times the mass of our Sun.
But how did a black hole of this amazingly large mass manage to evolve so soon after the Big Bang birth of the Universe, thought to have occurred almost 14 billion years ago? Astronomers have suggested that the primordial Universe might have had conditions that would permit the formation of black holes of incredibly enormous masses–reaching 100,000 times solar-mass. This is very different from the way black holes are born in the Universe today. The more recent generation of supermassive beasts rarely grow to be a few dozen times the mass of our Sun.
Dr. Bram Venemans of the Max Planck Institute for Astronomy in Germany, noted in a December 6, 2017 Carnegie Science Press Release that “Quasars are among the brightest and most-distant known celestial objects and are crucial to understanding the early Universe.
Supermassive black holes are the largest of their kind in the Cosmos. Black holes of “only” stellar mass are born when an extremely massive star blasts itself to pieces in the raging tantrum of a supernova explosion. A supernova marks the death of a star, that has ended its hydrogen-burning “life” on the main-sequence of the Hertzsprung Russell Diagram of Stellar Evolution. After a stellar mass black hole has formed from its own progenitor star’s funeral pyre, it can continue to grow by devouring its environment. Many astronomers think that by eating unfortunate stars and clouds of doomed gas, and by merging with others of its kind, the most massive black holes form. Astronomers have realized for years that probably every large galaxy in the Cosmos hosts a central supermassive beast in its heart, hiding there in voracious secret. These enormous gravitational monsters are bewildering, bewitching, and mysterious entities. This is primarily because they apparently already existed when the Universe was young.
Blobs of ill-fated gas and the shredded remains of unfortunate stars whirl down into the turbulent maelstrom surrounding supermassive black holes. This tragic buffet creates an enormous disk encircling the hungry black hole, termed an accretion disk. The tumbling banquet becomes hotter and hotter, as it hurls out radiation, as it flies ever closer and closer to the point of no return called the event horizon. The event horizon is located at the innermost portion of the accretion disk.
Quasar, Quasar Burning Bright
Many supermassive black holes were apparently already present in the primeval Universe, hungrily haunting the secretive hearts of the most ancient galaxies. These bizarre gravitational tigers reveal themselves as quasars to the prying eyes of curious observers. Quasars are actually the ferociously glaring accretion disks swirling brilliantly around these voracious, active supermassive beasts. These shining objects are vigorous, youthful Active Galactic Nuclei (AGN), that are powered by infalling material from the accretion disk. Some astronomers search for distant objects inhabiting the Universe, that sparkled like a myriad of celestial glow worms, when the baby Cosmos was still in the earliest stages of its existence. Quasars–or quasi stellar objects–are like dazzling celestial glow worms that ignited very long ago–and far away.
In astronomy, time and distance, as well as the wavelength of light at which the observations are being conducted, are all closely related. Because light zips through Space at a finite speed and, as a consequence, inevitably must take a finite amount of time to reach our waiting telescopes, objects that are very far away are seen the way they were in the distant past. Astronomers use what is termed the redshift (z) to discover how long ago and far away a luminous cosmic object is. The measurable quantity 1+z is the factor by which the Universe has expanded–between the long ago era when a distant, ancient source first tossed its light out into Space, and the current Time, when it at last could be observed by Earthly astronomers. The redshift also indicates the factor by which the wavelength of traveling light now approaching us has been stretched as a result of the expansion of the Universe–it is the shift of a luminous object’s spectrum toward longer and longer wavelengths–or towards the red end of the electromagnetic spectrum, as it zips away from us.
Supermassive tigers and their encircling, glaring accretion disks can be–at least–as vast as our entire Solar System. These gravitational beasts are described by their heavy weight, greed, hunger, and repulsive table manners. When its outside source of energy at last runs out, the quasar switches off. The best current calculations indicate that most galaxies went through a quasar stage in the ancient Cosmos, and that they now contain a remnant, often dormant, supermassive black hole that exhibits a mere shadow of its former youthful appetite. This is thought to be how the supermassive hole that haunts the heart of our own Milky Way Galaxy evolved through Time. As supermassive black holes go, our Galaxy’s resident beast is a small one, with a small appetite–and it is “merely” millions, as opposed to billions, of solar-masses. Once, very long ago, our Galaxy’s supermassive black hole dazzled the ancient Cosmos as a brilliant quasar–but it is a quiet old tiger now, except for those rare occasion when it goes on an eating binge, and swallows an enormous helping of shredded stars and/or disrupted clouds of gas that unfortunately wandered too close to its waiting maw. Our Milky Way’s resident beast has been named Sagittarius A* (Sagittarius-a-star), and it is tired out, as it slumbers peacefully in its old age–except when it now and then wakes up and feasts on its wandering prey with the greedy hunger of its youth–but only for one brief shining moment.
History Of The Hunt
In the 18th century, John Michell and Pierre-Simon Laplace predicted the existence of massive black holes. Albert Einstein’s Theory of General Relativity (1915) predicted the existence of bizarre objects with such deep gravitational wells that anything unlucky enough to travel too close to their waiting maws would be devoured. However, the prospect of the true existence in nature of such graviational beasts seemed so preposterous at the time that even Einstein at first rejected the idea–even though his own calculations showed otherwise.
In 1916, Karl Schwarzschild derived the first modern solution of the Theory of General Relativity that was able to describe a black hole. However, its interpretation as a portion of space, from which absolutely nothing could escape, was not well understood for another forty years. For decades, black holes were thought to be mere mathematical oddities, and it was not until the 1960s that theoretical work demonstrated that black holes are a generic predictton of General Relativity.
Dr. Donald Lynden-Bell and Dr. Martin Rees proposed back in 1971 that the center of our Galaxy would host a supermassive black hole. Sagittarius A* was discovered and given a name on February 13 and 15, 1974, by astronomers Dr. Bruce Blalick and Dr. Robert Brown, using the baseline interferometer of the National Radio Astronomy Observatory (NRAO). They discovered a radio source that emits synchrotron radiation–as well as being both immobile and dense because of its powerful gravitation. This proved to be the first hint that a supermassive black hole haunts the heart of our Milky Way.
The origin of supermassive black holes is a mystery. However, most astrophysicists agree that once a black hole has invaded a galaxy’s center, it can grow by accreting more and more matter from its surroundings, as well as by merging with other black holes. Several hypotheses have been proposed to explain the formation mechanisms of these strange supermassive hearts of darkness, in addition to the original masses of their progenitors–the “seeds” that grow in size to become enormous black holes of millions to billions of solar-masses. One of the most apparent explanations is that the “seeds” are actually smaller black holes of tens–or maybe hundreds–of solar-masses. These smaller black holes of stellar mass are the relics left behind by the supernova blasts that tore a massive star to pieces. These primordial objects then went on to merge and accrete additional matter. A second model suggests that an immense cloud of gas, that floated around in the ancient Cosmos before the first stars were born, collapsed to create a “quasi-star”, and then a black hole. This smaller black hole then quickly gained sufficient mass to become an intermediate-mass black hole–or possibly a supermassive one, if the accretion-rate did not shut off at higher masses. The initial “quasi-star” would be unstable to radial perturbations. This is because of electron-positron production in the core. A positron is an electron’s anti-matter twin, possessing a positive charge, as opposed to an electron’s negative charge. This “quasi-star” may finally collapse directly into a black hole, without a supernova explosion, which would hurl most of its mass into Space, and prevent it from leaving behind a black hole as a tattle-tale remnant.
A third scenario evokes a dense stellar cluster experiencing core-collapse as the negative heat capacity of the system shoots the velocity dispersion in the core to relativistic speed. A fourth model proposes that primordial black holes that may have been born directly from external pressure in the first moments following the Big Bang birth of the Universe almost 14 billion years ago. Black hole birth from the deaths of the first generation of stars has been both studied and corroborated by observations. A fifth proposal states that all of the other models for black hole formation–mentioned above–are merely theoretical.
The problem inherent in forming a supermassive beast is that there is a necessity for enough matter to be squashed into a small enough space. This matter must have very little angular momentum in order for this to occur. Usually, the process of accretion requires moving a large initial amount of angular momentum outwards, and this is thought to be a limiting factor in black hole growth. This is one of the most important components of the theory of accretion disks. The accretion of gas is considered to be the most efficient way–as well as the most conspicuous way–in which smaller black holes grow larger. Most of the mass growth of supermassive black holes is generally believed to occur through episodes of speedy gas accretion, which can be seen as AGN–or quasars. Observations reveal that quasars were much more abundant when the Universe was young. This suggests that supermassive tigers formed and grew early in the ancient Universe. Indeed, the existence of these distant, dazzling quasars indicates that supermassive black holes of billions of solar masses had already formed when the Universe was less than one billion years old. This means that these gravitational beasts were born in the primordial Universe, tucked within the hearts of the first massive galaxies.
The Banados quasar is particularly fascinating because it is from the time termed the epoch of reionization, which was the ancient era when the Universe emerged from its mysterious dark ages–and dancing light was finally free to wander its lovely way through a now-transparent Cosmos.
The Most Distant Dark Heart
The Big Bang started the Universe off as a searing-hot, murky, witch’s cauldron of extremely energetic particles that was very rapidly expanding. As it expanded, it cooled. Approximately 400,000 years later–a blink of the eye on the Cosmic time scale–these particles cooled and then coalesced to create neutral hydrogen gas. The Universe remained dark, devoid of dazzling sources of light, until gravity condensed matter into the first generation of brilliant fiery baby stars and newborn galaxies. The energy liberated by these primeval galaxies caused the neutral hydrogen, that swirled throughout the Cosmos, to become excited and ionize–meaning that the neutral hydogen atoms lost an electron, a condition that it has remained in ever since. Once the Universe became reionized, the liberated photons (packets of light) could fly freely through the previously dark Cosmos.
The new analysis of the Banados quasar reveals that a large percentage of the hydrogen in its nearby neighborhood is neutral. This means that the astronomers have discovered a source that existed in the very ancient epoch of reionization–long before the first stars and galaxies switched on to fully reionize the Universe.
“It was the Universe’s last major transition and one of the current frontiers of astrophysics,” Dr. Banados noted in the December 6, 2017 Carnegie Science Press Release.
It took more than 13 billion years for the traveling light emanating from the Banados quasar to reach us. The investigation of the quasar’s host galaxy was conducted using the IRAM/NOEMA and JVLA interferometer. The new findings are reported in a companion article published in The Astrophysical Journal Letters led by Dr. Bram Venemans.
“This great distance makes such objects extremely faint when viewed from Earth. Early quasars are also very rare on the sky. Only one quasar was known to exist at a redshift greater than seven before now, despite extensive searching,” commented Dr. Xiaohui Fan in the December 6, 2017 Carnegie Science Press Release. Dr. Fan is of the University of Arizona’s Steward Observatory.
Between 20 and 100 quasars as remote and brilliant as the quasar discovered by Dr. Banados and his team are predicted to exist over the whole sky. For this reason, it is a major discovery that will provide important information about the primordial Cosmos when it was only 5% its current age.
“This is a very exciting discovery, found by scouring the new generation of wide-area, sensitive surveys astronomers are conducting using NASA’s Wide-field Infrared Survey Explorer in orbit and ground-based telescopes in Chile and Hawaii. With several next-generation, even-more-sensitive facilities currently being built, we can expect many exciting discoveries in the very early Universe in the coming years,” said Dr. Daniel Stern in the Carnegie Science Press Release. Dr. Stern is of NASA’s Jet Propulsion Laboratory in Pasadena, California.
The team of astronomers used a duo of Magellan Telescope instruments to observe the supermassive black hole; FIRE, which made the discovery, and Fourstar, which was used to obtain additional images.
Las Campanas Director Dr. Leopoldo Infante noted in the December 6, 2017 Carnegie Science Press Release that “This important discovery–together with the detection of distant galaxies–is elucidating the conditions of the Universe during the reionization epoch. While we wait for the construction of the new generation of giant telescopes, such as the GMT, telescopes such as the Magellans at Las Campanas Observatory in Chile will continue to play a crucial role in the study of the early Universe.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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