Undernourished Black Hole Sends Its Dazzling Galaxy Into The Darkness

They dwell hungrily and secretively in the strange dark hearts of most, if not all, of the large galaxies that perform their mysterious and bewitching dance throughout the vast expanse of the observable Universe. They are supermassive black holes, and these weird objects can be millions to billions of times more massive than our Sun. Supermassive black holes are the largest form of their exotic kind, and they possess bizarre properties that separate them from lower mass black holes of “only” stellar mass. Many galaxies have been observed to have a dazzling, brilliant core that is powered by a supermassive beast, and these glaring cores make “active galaxies” some of the brightest inhabitants of the Universe. In September 2016, an international team of astronomers announced that they have solved the intriguing mystery of a rare alteration in the behavior of a supermassive black hole lurking at the center of a distant galaxy. It seems that this strange dark heart has fallen on hard times and is no longer able to feed itself an adequate diet of nourishing fuel to make its surroundings shine brilliantly across Space and Time.

The supermassive black hole inhabitants of the Cosmos are thought to glare brightly because searing-hot material is shining ferociously as it tumbles down, down, down into the waiting gravitational jaws of the beast–in a process termed accretion. This dazzling light can differ greatly between individual active galaxies, and for this reason astronomers classify them into several types based on the properties of the light that they cast out into the Universe.

Some of these galaxies have been observed to change dramatically over the passage of only a decade–a mere wink of the eye on cosmological time scales. However, the active galaxy of this new study, dubbed Markarian 1018, stands out in the crowd because it was caught changing its type a second time. The supermassive beast was caught reverting back to its initial classification within only the last five years. Only a very small number of galaxies have been observed to make this strange full cycle sea-change–but never before has one been observed in such great detail.

The Nature Of The Beast

Although many of the mysteries concerning the properties of these bewitching supermassive objects remain to be solved by curious scientists, they are nonetheless known to be some of the most bizarre entities dwelling in our indisputably weird Universe. Gravitational forces close to these dark hearts are so fiercely powerful that nothing–nothing, nothing, nothing at all–can escape from the snatching claws of these powerful gravitational monsters. Smaller stellar mass black holes form in the catastrophic supernova explosions that mark the violent “deaths” of massive stars. However, black holes of the supermassive variety are believed to be born differently from their smaller relatives. How these enormous gravitational beasts are born, and their role in the formation and evolution of the galactic structures that host them, are questions that a large number of astrophysicists are currently trying to answer.

Once a supermassive heart of darkness is situated in the core of a galaxy, it can continue to acquire more and more mass by devouring its surroundings, and by merging with others of its own bizarre kind. Shredded stars and clouds of gas swirl into the merciless, turbulent maelstrom surrounding these bizarre objects. This tumbling buffet of swirling material forms an enormous accretion disk encircling the supermassive black hole. This infalling, swirling material becomes ever hotter and hotter and emits radiation, especially as it travels ever closer and closer to the infamous point of no return called the event horizon. The event horizon is located at the innermost portion of the accretion disk.

Supermassive black holes, along with their circling and swirling accretion disks, can be (at least) as large as our whole Solar System. Furthermore, these insatiably hungry bizarre entities can be described by their enormous appetites, messy eating habits, and heavy masses. Black holes of much smaller stellar mass are born when a massive star collapses and perishes in the brilliant rage of a supernova explosion. The unfortunate and doomed massive star has reached the catastrophic end of that long stellar road because it has run out of its necessary supply of nourishing nuclear-fusing fuel, and it is about to go supernova. After the blast, a black hole forms from the fiery funeral pyre of its progenitor star–and it can go on gaining weight by eating whatever wanders too close to where it lies in sinister secret. The growing black hole dines greedily on stars and gas, but it also merges with other black holes. Some astronomers think that this is how supermassive black holes form.

In the 18th century, John Michell and Pierre-Simon Laplace predicted the existence of these strange gravitational monstrosities. Albert Einstein, in his Theory of General Relativity (1915) predicted the existence of bizarre objects bearing such powerful gravity that anything unfortunate enough to wander too close to their gravitational claws would be swallowed. However, the notion that such strange objects could really exist in nature seemed so far-fetched at the time that Einstein at first doubted his own prediction–even though his calculations indicated otherwise.

In 1916, Kark Schwarzschild derived the first modern solution to the Theory of General Relativity that could describe a black hole. However, for decades, these strange objects were considered to be only mathematical oddities that could not really exist in the Universe. It was not until the 1960s that theoretical studies showed that black holes are a generic prediction of General Relativity. Place a large enough amount of matter into a sufficiently small space, and a black hole will form every time!

In 1971, the English astrophysicists Donald Lynden-Bell and Martin Rees presented their hypothesis proposing that the heart of our own large barred-spiral Milky Way Galaxy could harbor a supermassive black hole. Sagittarius A*, our Galaxy’s heart of darkness, was discovered and named on February 13 and 15, 1974, by the American astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory (NRAO), a Federally Funded Research and Development Center of the U.S. National Science Foundation established for the purpose of conducting radio astronomy. Balick and Brown detected a radio source that emits synchrotron radiation–and it was observed to be both dense and immobile as a result of its gravitation. This was the first suggestion that a supermassive black hole haunts our own Galaxy’s core. Sagittarius A* is quiet now. It is an elderly dark heart of mere millions of solar masses, which makes it a relative light-weight as far as supermassive black holes go. Many supermassive dark hearts, lurking in the cores of the vast multitude of the Universe’s galaxies, weigh billions–as opposed to millions–of solar-masses.

While a great deal is known about the supermassive dark hearts that lurk hungrily in the cores of the myriad galaxies inhabiting our Universe, their origin remains an open field of research. Many astrophysicists think that the most obvious theory explaining their birth is that they form from mergers of their stellar mass kin. Stellar mass black holes weigh-in at tens or perhaps hundreds of solar-masses, and they are the relics of massive stars that have gone supernova. Black holes then continue to grow by the accretion of more and more matter–eventually becoming supermassive.

A second scenario involves a large cloud of pristine gas existing in the primordial era long before the first generation of stars were born. According to this model the huge gas cloud would collapse into what is termed a quasi-star— and then evolve into a black hole of only about 20 solar-masses. However, this “small” object would then rapidly start to accrete more and more matter, growing relatively rapidly into a black hole of intermediate-mass, and possibly eventually into a supermassive heart of darkness if the accretion rate is not quenched at greater masses. The progenitor quasi star would grow unstable to radial perturbations as a result of electron-positron pair production in its core, and may collapse directly into a black hole without the need for a supernova blast, which would shoot out most of its mass and prevent it from leaving a black hole behind as a relic to tell its sad story to the Universe.

There is yet a third scenario that explains how a supermassive gravitatonal beast may be born. According to this model, a dense cluster of stars would experience core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds.

A fourth model suggests that primordial black holes may have been born directly from external pressure in the first moments of the Big Bang birth of the Universe almost 14 billion years ago.

The emergence of black holes from the funeral pyres of massive stars that have gone supernova has been extensively studied and corroborated by observations. However, the other models remain theoretical.

Undernourished Black Hole Sends Its Dazzling Galaxy Into The Darkness

The observation that Markarian 1018 has something of an odd and fickle nature was made by chance. This new discovery came as a result of the Close AGN Reference Survey (CARS), which is a collaboration between the European Southern Observatory (ESO) and other groups that collect information about 40 nearby galaxies that display bright, glaring cores. The CARS team used the ESO’s Very Large Telescope (VLT), along with the NASA/European Space Agency’s (ESA’s) Hubble Space Telescope (HST), and NASA’s Chandra X-ray Observatory. Routine observations of Markarian 1018 with the Multi-Unit Spectroscopic Explorer (MUSE) installed on ESO’s VLT uncovered the intriguing alteration in the light output of the galaxy.

“We were stunned to see such a rare and dramatic change in Markarian 1018,” commented Rebecca McElroy in a Setpember 15, 2016 ESO Press Release. She is lead author of the discovery paper and a doctoral candidate at the University of Sydney and the ARC Centre of Excellence for All Sky Astrophysics (CAASTRO) in Australia.

The unexpected chance observation of the galaxy, so soon after it had begun to fade away, provided a precious opportunity for the astronomers to learn what makes these galaxies do the things they do. “We were lucky that we detected the event just 3-4 years after the decline started so we could begin monitoring campaigns to study details of the accretion physics of active galaxies that cannot be studied otherwise,” noted Bernd Husemann, CARS project leader and lead author of one of the papers describing the discovery. He is of ESO Headquarters near Munich, Germany.

The team of astronomers took advantage of this unexpected opportunity, and made it their highest priority–using it to detect the process causing Markarian 1018’s wild brightness alterations. The changes in brightness could have been caused by any one of a number of astrophysical events, and the astronomers needed to rule out the possibility of the black hole pulling in and devouring a single star, as well as the possibility of obscuration by intervening gas. Nevertheless, the real mechanism behind the observations of Markarian 1018’s surprising alterations remained mysterious following the first round of observations.

However, the astronomers were able to collect extra data after they were awarded observing time to use the HST and the Chandra X-ray Observatory. With the newly acquired data derived from these instruments they were able to solve the mystery–the supermassive black hole lurking in the dark heart of Markarian 1018 was slowly fading because it was starving to death.

“It’s possible that this starvation is because the inflow of fuel is being disrupted. An intriguing possibility is that this could be due to interactions with a second supermassive black hole,” noted McElroy. Such a black hole binary system is a definite possibility in Markarian 1018. This is because the galaxy is the product of a catastrophic major collision and merger between two individual galaxies. In this scenario, each member of the galactic duo contained its own resident supermassive beast haunting its center.

Studies are ongoing into the mechanisms that are at work in active galaxies, like Markarian 1018, that cause them to change their appearance. Husemann noted in the September 15, 2016 ESO Press Release that “The team had to work fast to determine what was causing Markarian 1018’s return to the shadows. Ongoing monitoring campaigns with ESO telescopes and other facilities will allow us to explore the exciting world of starving black holes and changing active galaxies in more detail.”

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