Reclusive Galaxy With A Big Dark Heart

Lurking in the dark hearts of most, if not all, large galaxies, they wait in sinister, predatory secret for their dinner–a tasty morsel of a nomadic, and by now shredded star, perhaps, or an unlucky cloud of doomed gas, that has wandered too close to their powerful gravitational snatching claws. These gravitational monsters are supermassive black holes, and they can weigh millions to billions of times more than our Sun. In the ancient and distant regions of our Universe, supermassive beasts power brilliant quasi-stellar-objects, called quasars, which are powerful beacons of fierce, brilliant, and raging light whose energy is emitted as a result of the accretion of gas onto the massive black holes–causing them to frequently outshine their entire host galaxy by many orders of magnitude. Until now, the dormant descendants of glaring quasars have typically been discovered in giant galaxies situated at the centers of massive galaxy clusters containing hundreds of other galactic constituents–but what has become of all the other accreting supermassive black holes? In April 2016, astronomers announced that one of them–and an extremely massive one, at that–has been found, but in an unexpected place, hiding in the center of a galaxy inhabiting a sparsely populated region of the Universe.

Indeed, within the dark heart of a galaxy named NGC 1600, there lurks a giant black hole sporting the almost incredible, and unimaginable, mass of 17 billion times that of our Sun. This is clearly one of the largest supermassive beasts discovered to date! The observations, made by NASA’s venerable Hubble Space Telescope (HST) and the Gemini telescope in Hawaii, could indicate that these dark behemoths may be more common denizens of the Cosmos than previously thought.

Until this study, the largest supermassive beasts–those that are approximately 10 billion times solar-mass–have been discovered inhabiting the dark hearts of giant galaxies in regions of the Universe that are heavily populated by other large galaxies. In fact, the current record-holder weighs-in at an astounding 21 billion suns, and dwells within the very crowded Coma cluster of galaxies, which contains more than 1,000 galactic constituents.

“The newly discovered supersized black hole resides in the center of a massive elliptical galaxy, NGC 1600, located in a cosmic backwater, a small grouping of 20 or so galaxies,” Dr. Chung-Pei Ma commented in an April 6, 2016 Hubblesite Press Release. Dr. Ma, the lead discoverer of NGC 1600’s immense dark heart, is an astronomer at the University of California-Berkeley, and head of the MASSIVE Survey, which is a study of the most massive galaxies and supermassive black holes in our large spiral Milky Way Galaxy’s local neighborhood, being only about 350 million light-years away. Even though the discovery of similar gigantic black holes in a massive galaxy inhabiting a crowded domain of the Cosmos is not surprising, it has previously been considered unlikely that they could be found within less heavily populated portions of the Universe.

“There are quite a few galaxies the size of NGC 1600 that reside in average-size galaxy groups. We estimate that these smaller groups are about 50 times more abundant than spectacular galaxy clusters like the Coma cluster. So the question now is, ‘Is this the tip of an iceberg?’ Maybe there are more monster black holes out there that don’t live in a skyscraper in Manhattan, but in a tall building somewhere in the Midwestern plains,” Dr. Ma added.

The new study shows that the most massive black holes are not limited to the regions of highest density in the Universe. The international team of astronomers who made this important discovery are from the United States, Canada, and Germany. The astronomers were analyzing observations obtained from a survey of early type massive galaxies, when they discovered the behemoth black hole situated at the center of the group galaxy NGC 1600. In particular, the astronomers measured the velocities of stars near the supermassive beast which were then inserted into models for stellar orbits in order to determine the mass of the black hole.

The huge mass of the black hole, combined with the fact that NGC 1600 is a member of a relatively small group composed of only a few galaxies, makes this discovery particularly exciting: “This is the first time that we find such a massive black hole in a relatively isolated galaxy, outside a rich galaxy cluster. Other galaxies found to harbor very massive black holes are typically located in dense regions of the Universe populated by many other galaxies and clusters,” explained Dr. Jens Thomas in an April 6, 2016 Max Planck Institute for Extraterrestrial Physics (MPE.MPG) Press Release. Dr. Thomas, who is of the MPE in Garching, Germany, is lead author of the study published in the April 6, 2016 issue of the journal Nature.

NGC 1600 is the brightest galaxy inhabiting its group, and it out-dazzles all of the other galactic constituents at least three times over. In order for this behemoth black hole to have grown so enormous, it likely had a head start, merging with its former neighboring galaxies and their central black holes in the early Universe. According to this scenario, it did so during an ancient era when galaxy interactions were more frequent in a considerably smaller and crowded Cosmos. When two galaxies bump into one another and then merge, their duo of dark hearts settle into the center of the new galaxy and orbit each other. Stars tumbling down near this newly-formed black hole binary, depending on the speed of their trajectory, can actually steal away momentum from the whirling duo of black hole dancers and pick up sufficient velocity to escape to freedom away from the galactic core. This strange gravitational waltz results in the black holes gradually dancing ever closer and closer together. Eventually, the duo merges to form an even larger black hole. The supermassive beast then goes on to grow bigger and bigger by feasting on gas that is funneled to the core by galaxy collisions. “To become this massive, the black hole would have had a very voracious phase during which it devoured a lot of gas,” Dr. Ma explained in the April 6, 2016 Hubblesite Press Release.

The Dancers And Their Dance

The possibility of the real existence of black holes dancing around in our Cosmos was first proposed in the 18th century by John Michell and Pierre-Simon Laplace, who made the prediction of such gravitational monstrosities. Albert Einstein, in his Theory of General Relativity (1915) went on to predict the existence of weird objects possessing such deep and inescapable gravitational wells that anything that was unlucky enough to travel too close to their predatory, waiting maws would be gobbled up. However, the prospect of the real existence in Nature of such oddities seemed so outrageous at the time that even Einstein initially rejected the idea–even though his own calculations were shouting otherwise.

In 1916, Karl Schwarzschild derived the first modern solution to Einstein’s Theory of General Relativity that could define a black hole. However, its interpretation as a place in space from which absolutely nothing, nothing, nothing at all could ever escape, was not sufficiently grasped for another forty years. For decades, black holes were considered to be only a mathematical oddity. It was not until the 1960s that theoreticians demonstrated that these strange objects are a true generic prediction of General Relativity.

Supermassive black holes have smaller cousins. Black holes of “only” stellar mass are born when a very massive star collapses and perishes in the fiery tantrum of a brilliant, fatal Type II (core-collapse) supernova. Supernovae mark the tragic end of a star’s “life” on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of stellar evolution. After a black hole has formed amid the wreckage of this catastrophic and magnificent stellar funeral pyre, it can continue to gain even more mass by feasting on its environment. Many scientists think that by gobbling up unfortunate stars, tragic clouds of wandering gas, and by merging with others of its strange kind, the most massive of black holes–the supermassive kind–are born. Astronomers have understood for years that probably every large galaxy in the Cosmos contains a central supermassive beast, secreted away in predatory wait, in their hearts of darkness. These gigantic gravitational monsters are bewitching and bewildering objects–possibly because they were already hanging around in space when the Universe was young.

The infalling banquet, that feeds a voracious dark-hearted beast, swirls down into the violent, roaring maelstrom of this bizarre object’s gravitational grasp. This swirling, whirling buffet of starry stuff, gas, and whatever else has proven to provide a convenient lunch, grows hotter, and hotter, and hotter until it hurls out radiation–especially as it moves ever closer to that bizarre point of no return termed the event horizon. The event horizon is situated at the innermost region of the accretion disk. The searing-hot and glaring accretion disk forms from the material that is whirling down into the black hole’s waiting maw.

Because light travels at a finite speed, it takes a finite amount of time to reach us. In astronomy, time, distance, and the wavelength of light are all bound together. Because light travels at a set speed, very remote objects are observed the way that they appeared in the distant past, because the light from very faraway objects has taken a longer time to reach us than the light traveling to us from objects that are relatively close. Astronomers use the redshift (z) to determine how long ago and far away a celestial object is. The measurable quantity of 1 + z is the factor by which the Universe has expanded between the time when a distant source first cast its traveling light out into space, and the current time, when it can be observed. Furthermore, it is also the factor by which the wavelength of light now traveling towards us has been stretched by the expansion rate of the Universe. The redshift is the shift of a luminous object’s spectrum toward ever longer wavelengths–or towards the red end of the electromagnetic spectrum, as it travels away from us. Conversely, the light from objects that are traveling towards us are shifted to the blue end of the electromagnetic spectrum (blue shift).

Supermassive black holes and their accompanying glaring and surrounding accretion disks, can be as big as our entire Solar System. These gravitational behemoths are described by their immense weight, voracious appetites, and sloppy table manners. When its outside source of energy is finally used up, the quasar switches off. The latest estimates indicate that most galaxies went through a quasar phase when our Universe was young, and that they currently contain relic, usually dormant, supermassive black holes that display only a shadow of their former insatiable appetites. This model possibly illustrates the way that the supermassive black hole inhabiting the dark heart of our Milky Way Galaxy evolved. As supermassive black holes go, it is one of the smaller ones, with a mass of a “mere” 4 million Suns–as opposed to the billions of solar-masses boasted by some of its larger relatives. Once, a very long time ago, when the Universe was young, our Milky Way’s resident heart of darkness likely lit up the primordial Cosmos as a dazzling quasar. However, it is dormant and quiet now, except on those rare occasions when it lights up like it did long ago, and goes on an eating binge–hungrily devouring an infalling buffet of shredded stars and/or messed up clouds of gas that have tragically wandered too close to this waiting, predatory gravitational beast. Our Milky Way’s supermassive black hole is called Sagittarius A* (Sagittarius-a-star), and it is calm and peaceful in its dotage, except for when it blasts the Universe with its forgotten fires, feasting on its prey, with the dazzling and insatiable hunger of its flaming youth–but only for one brief shining moment.

Reclusive Galaxy With A Big Dark Heart!

The frequent snacks devoured by NGC 1600 may be the explanation for why this strange and glaring galaxy dwells so far from the madding crowd, with only a few galactic neighbors to associate with. Indeed, NGC 1600 is the most dominant galaxy of its galactic group. This is because it is at least three times more brilliant than its neighbors. “Other groups like this rarely have such a large luminosity gap between the brightest and the second brightest galaxies,” Dr. Ma commented in the April 6, 2016 Hubblesite Press Release.

In fact, most of NGC 1600’s gas was consumed long ago when the black hole blazed with brilliance as a dazzling hot young quasar–lighting up the ancient Cosmos, as a result of the material screaming into it, that was heated up into a glaring plasma. “Now, the black hole is a sleeping giant. The only way we found it was by measuring the velocities of stars near it, which are strongly influenced by the gravity of the black hole. The velocity measurements give us an estimate of the black hole’s mass,” Dr. Ma continued to explain.

The velocity measurements were obtained by the Gemini Multi-Object Spectrograph (GMOS) sported by the Gemini North 8-meter Telescope poised atop the Mauna Kea dormant volcano in Hawaii. GMOS spectroscopically separated the light from the galaxy’s heart, unveiling stars within 3,000 light-years of the core. Some of these stars are orbiting around the black hole and evading close encounters of the worst kind. Alas, stars traveling on a straighter path away from the core indicate that they had wandered too close to the hungry giant and had been unceremoniously tossed away–probably by the black hole duo of long ago.

Archival HST images, obtained by the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), strengthen the theory of twin black holes hurling the stars away. The NICMOS images showed the galaxy’s core was unusually dim, suggesting a lack of stars near the galactic center. A core barren of sparkling, fiery stars distinguishes massive galaxies from the more standard elliptical (football-shaped) galaxies, which are considerably brighter in their centers. Dr. Ma and her team estimated that the number of stars hurled out of the central region is equivalent to 40 billion suns–comparable to ejecting the entire disk of our Milky Way Galaxy!

Dr. Jens Thomas explained in the April 6, 2016 MPE.MPG Press Release that “Equally astonishing is the center of the galaxy: it is very diffuse, as if billions of stars are missing… Less massive elliptical galaxies typically get brighter and brighter the closer you get to the center, but in NGC 1600 it’s like the equivalent of all the stars of the Milky Way’s disk have been removed.”

By comparing their findings with mass determinations of a sample of other core galaxies, the astronomers discovered that the radius of the region with depleted stellar densities is indistinguishable from the gravitational sphere of influence of the supermassive beast. The core radius seems to be a better indicator of black hole mass than other galaxy properties.

Dr. Ma told the press on April 6, 2016 that “The black hole in NGC 1600 is the first example of a possible descendant of a luminous quasar in a relatively isolated galaxy. There are quite a few galaxies of comparable size that reside in average-sized galaxy groups. At the moment we do not know if such very massive black holes are common in other nearby massive galaxies as well. Our ongoing observations will soon reveal if our discovery is a rare find or just the tip of an iceberg.”

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