Our Galaxy’s Supermassive Black Hole Has Hidden Siblings
By Judith E Braffman-Miller
Strange, sinister, and hungry, supermassive black holes lurk in the secretive hearts of perhaps every large galaxy in the visible Universe. Our own Milky Way Galaxy’s resident dark heart is named Sagittarius A (pronounced Sagittarius-a-star), or Sgr A, for short. It is a peaceful, elderly gravitational beast now, only awakening from its slumber occasionally to devour an unfortunate tidbit–such as a disrupted star or cloud of doomed gas–that has wandered too close to its waiting maw. Supermassive black holes, such as Sgr A*, can sport impressive masses millions to billions of times more than that of our Sun. But our Galaxy’s resident old black hole is a relatively puny beast–weighing-in at “only” millions, as opposed to billions, of times solar-mass. Even though our own supermassive black hole resides within our Milky Way, it still harbors, deep within its hidden heart, many strange secrets. In April 2018, yet another one of its many mysteries was revealed when a team of astronomers announced their new findings that our Galaxy’s supermassive dark heart may have “unseen” siblings.
As a result of this new research, astronomers are now beginning to gain an understanding of what occurs when black holes develop wanderlust. Even though supermassive beasts typically dwell within the heart of a massive galaxy, such as our own, this is not always the case. Alas, these strange gravitational monsters sometimes get the urge to roam throughout their host galaxy, and during their travels come to lurk hungrily far from the galactic center, in distant regions such as the stellar halo. Stellar halos are nearly spherical areas of a galaxy that contain both stars and gas. These halos encircle the primary section of a galaxy.
Currently many astronomers propose that these wandering black holes are the result of mergers between galaxies in an expanding Cosmos. A smaller galaxy will sometimes join a larger member of its own kind and–as a result–deposit its own resident supermassive beast onto a wide orbit within the new (and much larger) host galaxy.
Sagittarius A* And Others Of Its Kind
The first generation of black holes to dwell in the Universe both created and destroyed. On the one hand, they ruthlessly devoured any object unlucky enough to travel too close to their waiting, voracious maws. On the other hand, they simultaneously created jets of high-energy particles and radiation, that they churned out as a result of their terrible table manners. The jets created by these gravitational monsters can be millions of light-years in length, and many astronomers think that they are the triggers that form successive generations of baby stars. Basically, this means that the first black holes to appear in the ancient Universe served as the “seeds” from which galaxies ultimately formed. According to this model, galaxies evolved to swirl majestically around their resident supermassive beast. Indeed, these primordial black holes were necessary ingredients in the mysterious recipe responsible for galactic evolution–and they still are. In our own Solar System, black holes are responsible for the birth of our Sun, Earth, and our very existence.
Shredded stars and disrupted clouds of gas form the wreckage that ultimately tumbles down into the turbulent maelstrom surrounding a supermassive beast. This shredded banquet of whirling material forms an enormous disk surrounding the hungry, gluttonous black hole. The disk itself, termed an accretion disk, becomes increasingly hotter and hotter as time goes by–especially as it is sucked into the merciless vortex close to the point of no return–the black hole’s event horizon. The event horizon, from which nothing can escape–not even light–is located at the innermost region of the glaring accretion disk.
Supermassive dark hearts, and their surrounding, swirling accretion disks, can be (at least) as large as our entire Solar System, and they are described by their huge masses, greedy appetites, and sloppy eating habits. The supermassive black holes that haunt the hearts of galaxies in the Universe today, like Sgr A*, are often dormant in their dotage–showing only a shadow of the former appetites that they once had when both they, and the Cosmos itself, were much younger.
Once, very long ago, Sgr A* dazzled the ancient Universe with its brilliant light as a hot young quasar. Quasars are the accretion disks that encircle especially voracious and active supermassive dark hearts, that dance the light fantastic in the primordial Universe. The quasars that lit up the ancient Cosmos were young, hot, and especially brilliant active galactic nuclei (AGN) that were powered by the unfortunate material tumbling inward from the encircling accretion disk. Astronomers search the skies for celestial objects that dazzled like fireflies when the Cosmos was still in its youth, and quasi stellar objects (quasars) are just such glaring ancient objects.
Sgr A is itself invisible to the curious eyes of astronomers. This is because, like all other black holes, it sends no energy into Space at all–and is totally dark. Our Galaxy’s dormant dark heart, like other old black holes, shows little of the greedy appetite of its young quasar stage. Indeed, in the case of Sgr A, its been a very long time between dinners. Sgr A* enjoyed its last big banquet approximately six million years ago, when it greedily ate a very unlucky, large, doomed and disrupted cloud of tumbling gas. After this terrible feast, the sloppy and now satisfied black hole emitted an enormous bubble of gas that was equal to millions of solar masses. This gas bubble now billows both below and above our Milky Way’s dark heart. These after-the-feast bubbles, termed Fermi Bubbles, were first detected by NASA’s Fermi Gamma-ray Space Telescope in 2010.
Black holes are compact, dense regions of Space, with such powerful gravitational snatching claws that nothing can escape from their gravitational kiss of death. Sgr A* has compressed the mass of 4.5 million Suns into a tiny area of Space in the heart of our Galaxy.
Clearly supermassive black holes are weird beasts inhabiting the celestial zoo. Sgr A* is surrounded by a sparkling cluster of newborn stars, some of which have had the bad luck of diving to within only a few billion miles of where the hungry beast waits for its supper.
Our Galaxy’s supermassive beast is quiet now, enjoying a peaceful old age. Like other old black holes it accretes at a very slow rate. This makes it difficult for astronomers to distinguish them from the dark galactic hearts in which they hide. Sgr A* provides an instructive gift–it is an exception to this frustrating rule. This is because curious astronomers can gain a much closer view of its comparatively gentle X-ray emission.
Black holes, in general, can be large or small–they do not come in only one size. These celestial objects can be defined as an area of Spacetime where the merciless pull of gravity has become so powerful that not even light can escape, once it has been snared. The pull of gravity has grown this intense because matter has been crushed into a very tiny space. Squeeze enough material into a small enough region, and a black hole will be born every time.
In addition to supermassive black holes, there are smaller stellar-mass black holes inhabiting the Universe. These relatively small black holes form in the funeral pyre of a very massive star that has come to the end of the stellar road. The erstwhile heavy star has collapsed in the furious fires of a supernova conflagration that has blasted it to oblivion. The supernova explosion heralds the end of a massive star’s fiery life on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. After a black hole of stellar mass has risen from the wreckage of its progenitor star, it can continue to gain more weight when it hungrily and greedily eats whatever doomed object is unlucky enough to wander too close to where it waits.
Our Supermassive Black Hole’s Secret Siblings
In the new study, published in the April 24, 2018 edition of the Astrophysical Journal Letters, astronomers from Yale University (New Haven, Connecticut), the University of Washington (Seattle), the Institut d’Astrophysique de Paris (France), and University College London (UK), predict that galaxies with a mass similar to that of our Milky Way should host several supermassive black holes.
In order to reach this conclusion, the scientists used a new, state-of-the-art cosmological simulation, called Romulus. Romulus helped the scientists predict the dynamics of supermassive black holes, inhabiting the hearts of galaxies, with better accuracy than previous supercomputer simulation programs.
“It’s extremely unlikely that any wandering supermassive black hole will come close enough to our Sun to have any impact on our Solar System. We estimate that a close approach of one of these wanderers that is able to affect our Solar System should occur every 100 billion years or so, or nearly 10 times the age of the Universe,” explained study lead author Dr. Michael Tremmel in an April 24, 2018 Yale University Press Release. Dr. Tremmel is a postdoctoral fellow at the Yale Center for Astronomy and Astrophysics.
Dr. Tremmel continued to note that since wandering supermassive black holes are predicted to dwell far from the centers of galaxies and outside of galactic disks, it is improbable that they will continue to accrete more gas. This is the reason why they are invisible. “We are currently working to better quantify how we might be able to infer their presence indirectly,” he added.
Study co-authors are Dr. Fabio Governato (University of Washington), Dr. Marta Volonteri (Institut d’Astrophysique de Paris), Dr. Andrew Pontzen (University College London), and Dr. Thomas Quinn (University of Washington).
The study is part of the Blue Waters computing project supported by the National Science Foundation and the University of Illinois.
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 some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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