Galaxies And The Supermassive Black Holes They Host
By Judith E Braffman-Miller
In the deepest, darkest hearts of perhaps every large galaxy in the Universe, including our own Milky Way, supermassive black holes lie in secret, waiting for their dinner–a wrecked star, perhaps, or a cloud of tragic gas. These gravitational beasts are some of the strangest occupants of the Cosmic zoo, and they possess almost unimaginable masses of millions to billions of times that of our Sun. Over the years, astronomers have come to the realization that these enormous black holes are inexorably linked to the galaxies that house them–hidden as they are within the brilliant, ferocious glare of an encircling accretion disk, made from the ruthlessly tattered objects that compose their terrible feasts. Indeed, these greedy gravitational monsters appear to have a direct correlation to the galaxies that shelter them, and researchers have calculated that the mass of these strange objects show a constant relation to their galactic hosts. Stars being born in galaxies appear to be influenced by the resident supermassive black hole, but the mechanism explaining how and why this occurs has been elusive–until now!
“Supermassive black holes are captivating. Understanding why and how galaxies are affected by their supermassive black holes is an outstanding puzzle in their formation,” commented Dr. Shelley Wright in a December 20, 2017 Keck Observatory Press Release. Dr. Wright is a University of California, San Diego Professor of Physics.
In a study published in the December 20, 2017 issue of The Astrophysical Journal, Dr. Wright, graduate student Andrey Vayner, and their colleagues examined the energetics surrounding the fierce and powerful winds generated by the brilliant, active supermassive black hole (quasar) residing at the center of the 3C 298 host galaxy, located about 9.3 billion light-years away.
“We study supermassive black holes in the very early Universe when they are actively growing by accreting massive amounts of gaseous material. While black holes themselves do not emit light, the gaseous material they chew on is heated to extreme temperatures, making them the most luminous objects in the Universe,” Dr. Wright continued to explain.
Studies Of Starlight
The majority of astronomers today think that our Universe was born in the wild exponential inflation of the Big Bang about 14 billion years ago. All that we are, and all that we can know, emerged at this time when all of the matter in the Universe, all of Space and Time, came into being.
Hundreds of millions of years passed before matter finally began to collect, ultimately creating enormous clouds that coalesced and collapsed under the relentless pull of their own gravity–giving birth to the first generation of stars to dance the light fantastic in the baby Universe. It is generally thought that the first stars were enormous, extremely hot, and very powerful. The first stars devoured vast quantities of material, and for this reason they “only” burned brightly for a few hundred million years. Massive stars live fast and die young, consuming their necessary supply of hydrogen fuel much faster than smaller stars. Smaller stars take “life” easy and, as a result, “live” much longer than their massive kin. For example, our own relatively small Star, the Sun, has been shining for about four and a half billion years–and will still burn brightly for another 5 billion years, before it swells up to a monstrous size, becoming an elderly, bloated, crimson red giant star.
The formation of baby stars (protostars) can be triggered by the gravity of a passing star or the arrival of pressure waves emanating from a supernova blast heralding the demise of a massive star. This stellar explosion causes instability by pushing on one side of the cloud. As it collapses, the natal cloud fragments into smaller and smaller pieces. Tucked within each of these fragments, gravity begins to emit heat energy and the fragment condenses into a rotating sphere composed of searing-hot gas–the baby protostar.
As time goes by, the temperature and intense pressure within the baby star become so extreme that a continuous thermonuclear explosion commences. With the onset of this powerful chain reaction, hydrogen begins to fuse into the next heavier atomic element–helium. The tremendous force of this continual, merciless, and relentless emission of energy pushes everything outward until it, at last, reaches an equilibrium with gravity. As a result, the cloud ceases to collapse. When the radiation that results from the internal explosion reaches the edge of the natal cloud, it flees into Space in the form of light. A star is born.
When the continual internal nuclear blast that powers a star has converted its hydrogen into helium, the star inflates and starts a new form of energy release by converting helium into carbon–and then carbon into oxygen followed by other increasingly heavier atomic elements as charted in the familiar Periodic Table.
Basically, this means that stars are essentially factories that create the atomic material comprising literally everything in our familiar world–ourselves included. Alas, when new fuel sources are used up at ever faster and faster rates, the star starts to manufacture iron in its core. This means that the star has come to the tragic end of that long stellar road, because iron cannot be burned as nuclear fuel, and the thermonuclear activity at the doomed star’s core starts to shut down. When the core no longer emits sufficient energy to stop the merciless, constant pulling crush of gravity that squeezes everything inward, the star collapses quickly and dramatically, in its final farewell performance to the Universe.
If the doomed, dying tragedy, that was once a star, contains more than three times solar-mass, the massive star’s core will keep shrinking, and shrinking, and shrinking, until at last it becomes infinitely small–but nevertheless still harboring all of the mass of the massive star that it once was. This is how a stellar mass black hole forms–the strange bottomless entity in the fabric of Spacetime itself.
The first generation of black holes both created and destroyed–gorging themselves on anything unfortunate enough to wander too close to their waiting, greedy maws–while, at the same time, forming jets of high-energy particles and radiation generated by their sloppy feeding frenzy. The jets produced can be millions of light-years in length, and many astronomers believe them to be the trigger that forms successive generations of stars. This means that the first generation of black holes served as the seeds of the galaxies that spin majestically around them. These ancient black holes were essential to galactic evolution (they still are) and, in the long run, to the birth of our Sun, Earth, and our very existence.
The wreckage of stars and tumbling clouds of gas whirl down into the violent, turbulent maelstrom surrounding black holes, and this infalling feast of swirling material creates a gigantic disk encircling the greedy black hole. This disk, called an accretion disk, grows progressively hotter and hotter as it shoots out radiation, particularly as it whirls ever closer to the point of no return termed the event horizon.. The event horizon, from which nothing, not even light, can escape from the black hole’s clutches, is situated at the innermost portion of the accretion disk.
As astronomers peer farther and farther into Space, they are also looking farther and farther back in Time. There is no way to locate an object in Space, without also locating it in Time. Hence, the term Spacetime. The more remote a shining object is in Space, the longer it has taken its traveling light to finally reach our telescopes. No known signal in the Universe can move faster than light in a vacuum, and the light streaming out from faraway objects in the distant Universe can travel to us no faster than this universal speed limit will allow. In the primeval Universe, a large number of supermassive black holes, shining brilliantly in the hearts of the most remote and ancient of galaxies, give themselves away in the form of quasars. Quasars are the brilliant accretion disks encircling particularly greedy and active supermassive hearts of darkness–they are vigorous youthful Active Galactic Nuclei (AGN) that are powered by material somersaulting inward from the surrounding accretion disks. Astronomers hunt for cosmic objects that ignited like fireflies when the Cosmos was still in its early formative stages, and quasi stellar objects (quasars) are just such sparkling celestial fireflies, shining brilliantly very long ago and far away.
In astronomy, Time and Distance, as well as the wavelength of light–at which a distant object is being observed–are all linked together. Light travels at a finite speed, and thus takes a finite amount of time to reach us. This means that astronomers observe remote objects the way they were in the distant past, and they look just like they did long ago–when they first sent their light rushing through Spacetime. Astronomers use what is termed the redshift (z) to determine how ancient and distant a luminous cosmic object is. The measurable quantity of 1 + z is the factor by which the Universe has expanded–between the long ago era when a remote, ancient object first hurled its fabulolus light out into intergalactic space, and the present time, when it is finally being observed. It is also the factor by which the wavelength of light, currently traveling towards us, has been stretched by the expansion of Spacetime. The redshift is the shift of a shining object’s spectrum towards ever longer wavelengths–or towards the red end of the electromagnetic spectrum, as it flies away from us. Conversely, when an object is traveling towards us, its light is blueshifted to the blue end of the electromagnetic spectrum.
Supermassive black holes and their accompanying, glaring accretion disks, can be as enormous as our entire Solar System–at least. These gravitational beasts are described by their immense weight, voracious appetites, and sloppy table manners. When its outside source of energy is at last used up, the quasar switches off. The best calculations, at this time, indicate that most galaxies went through a quasar stage in the primeval Cosmos, and that they presently play host to what is a dormant relic of the way they were. The supermassive black holes in today’s Universe show only the ghost of their former greedy appetites. This particular model illustrates the way our own Milky Way’s resident supermassive beast evolved. As supermassive black holes go, our Galaxy’s beast is a small one. Our Milky Way’s heart of darkness is a “mere” millions of solar-masses–and not the billions of solar-masses sported by many others of its strange kind. Once long ago, our old black hole probably dazzled the primordial Cosmos with its brilliant light as a glaring young quasar–but it is a peaceful old monster now, except when it occasionally goes on an eating binge, greedily devouring a large helping of stellar wreckage, or disrupted clouds of gas, that wandered too close to where it lay in wait for supper. Our Galaxy’s resident dark heart has been dubbed Sagittarius A* (Sagittarius-a-star), and it is quiet in its old age, except on those rare occasions when it feasts on its unfortunate prey with the insatiable greed of its younger days, when it was a fiery quasar lighting up the Universe.
Galaxies And The Supermassive Black Holes They Host
The University of California, San Diego team’s observations showed that the powerful winds rushing from the supermassive black hole roar through the entire host galaxy and play an important role in the growth of its stars.
“This is remarkable that the supermassive black hole is able to impact stars forming at such large distances,” commented Dr. Wright in the December 20, 2017 Keck Observatory Press Release.
Currently, neighboring galaxies display a galaxy mass that is tightly correlated with the mass of their resident supermassive black holes. Wright’s and Vayner’s research suggests that the 3C 298 galaxy is 100 times less massive than it should be, when the mass of its behemoth supermassive black hole is taken into consideration.
Basically, this observation means that the supermassive black hole mass is established long before its host galaxy, and potentially the energetics from the quasar are able to control the growth of its galactic host.
In order to conduct the study, the University of California, San Diego team utilized multiple state-of-the art astronomical facilities. The first of these was Keck Observatory’s instrument OSIRIS (OH-Suppressing Infrared Imaging Spectrograph) and its advanced adaptive optics (AO) system. An AO system enables ground-based telescopes to attain higher quality images by correcting for the blurring resulting from Earth’s atmosphere. The resulting images are as good as those obtained from space.
The second major facility was the Atacama Large Millimeter/submillimeter Array (ALMA), which is an international observatory in Chile that is able to spot millimeter wavelengths using up to 66 antennae in order to achieve high-resolution images of the gas surrounding the quasar.
“The most enjoyable part of researching this galaxy has been putting together all the data from different wavelengths and techniques. Each new data set that we obtained on this galaxy answered one question and helped us put some of the pieces of the puzzle together. However, at the same time, it created new questions about the nature of galaxy and supermassive black hole formation,” Vayner noted in the December 20, 2017 Keck Observatory Press Release.
Dr. Wright agreed, and noted that the data sets were “tremendously gorgeous” from both Keck Observatory and ALMA. Both offered a treasure trove of new information about the Universe.
These findings are the first results from a larger survey of remote quasars and their energetics.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. 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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