Ancient Black Holes That Grew Up Too Fast
By Judith E Braffman-Miller
Quasars are dazzling, luminous objects, that hide supermassive black holes deep within their mysterious, secretive hearts of darkness. Indeed, quasars are so brilliant that they are visible over great cosmic distances, as infalling, doomed material–snatched from ill-fated stars and unfortunate clouds of wandering gas–swirls down into the waiting, greedy maw of the voracious black hole, increasing the supermassive beast’s great, fiery, and beautiful brilliance. First observed back in the 1960s, quasars hurl out into intergalactic space their dazzling, magnificent light, that is as brilliant as a trillion stars–from a region that is smaller than our Solar System! In May 2017, astronomers using the W.M. Keck observatory in Hawaii, announced that they have discovered very young quasars that display a puzzling property: these precocious objects sport the impressive mass of approximately a billion suns, yet have been accumulating stolen matter for a mere 100,000 years. It is generally thought that quasars of that hefty mass should have needed to pull in matter–with their gravitational snatching claws–a thousand times longer than that. This creates a delightful cosmic mystery for scientific detectives to solve.
Hidden deep within the mysterious dark heart of perhaps every massive galaxy in the observable Universe–including our own barred-spiral Milky Way–there lurks a voracious supermassive beast. How these bizarre objects are born, and how they have managed to grow to become as massive as millions or even billions of suns, is an unanswered question. Some stages of vigorous baby black hole growth are highly visible to the prying eyes of curious astronomers: Whenever there are large quantities of gas swirling down into the supermassive beast’s waiting maw, matter in the direct vicinity of the black hole hurls out copious amounts of brilliant, blazing light. The black hole has intermittently evolved into a quasar, one of the most luminous objects in the Cosmos.
Astrophysicists from the Max Planck Institute for Astronomy (MPIA), in Germany, led by doctoral student Anna-Christina Eilers, have announced their discovery of a trio of brilliant young quasars that challenge the most widely accepted model explaining how baby supermassive black holes grow to their monstrous mature sizes. The trio of quasars are extremely massive, but should not have had enough time to collect that much mass. The discovery looked far back in time in order to understand ancient cosmic history. Because quasars are so extremely bright, they can be observed out to large distances. The team of astronomers observed quasars whose traveling light took almost 13 billion years to reach Earth. As a result, the observations reveal these quasars not as they are today, but as they were almost 13 billion years ago, less than a billion years after the Big Bang birth of the Universe about 13.8 billion years ago.
In astronomy, long ago is the same as far away. The deeper astronomers peer into Space, the farther back they are looking in Time. This is because a distant celestial object’s traveling light has taken a longer time to reach Earth as a result of the expansion of the Universe. There is no known signal that can travel faster than light in a vacuum. Indeed, the light flowing towards our planet, making its extremely long and dangerous journey from remote, ancient luminous objects dwelling in the Cosmos, can move no faster than this universal speed limit will allow. In the most distant, primeval Universe, a bizarre population of supermassive black holes reveal their mysterious, dazzling presence with the terrible beauty of brilliant quasars.
Quasars are very far away, lighting up the young hearts of primeval active galaxies. In fact, quasars hurl out as much as a thousand times the energy output of our entire Milky Way Galaxy, which hosts 200 to 400 billion stars. This radiation is emitted almost uniformly across the entire electromagnetic spectrum, ranging from X-rays to the far-infrared with a peak in the ultraviolet optical bands. A number of quasars are also powerful sources of radio emissions and gamma-rays.
Black holes are so dense, and contain so much mass, that even light cannot escape from their strong gravitational grip. The existence of black holes has been theorized for over two centuries, but in the past it was considered to be impossible to observe them directly. This is the reason why astronomers had no way to test their theories–until the Hubble Telescope (HST) made its appearance. The high resolution of the HST made it possible for astronomers to observe the effects of the irresistible gravitational attraction of these bizarre objects on their environment.
Despite their name, black holes are far from being mere empty space. Instead, they form when an enormous amount of matter is squeezed into an extremely small area–and they also come in different sizes. There are stellar mass black holes, which are approximately the same mass as our own Sun, that are born from the wreckage left when a very massive star explodes in the fiery fury of a supernova blast, after having come to the end of that long stellar road. The dying star’s core collapses as the outer gaseous layers of the erstwhile star are blown away–leaving behind only a small but extremely dense sphere to tell the tragic story of what was once a star that is a star no more.
Supermassive black holes, millions to billions of times the mass of our Sun, have more mysterious origins than their stellar mass cousins. These dark-hearted supermassive beasts dwell in secretive, sinister splendor in the centers of galaxies.
The Link Between Black Holes And Quasars
Quasars were once considered to be lonely, isolated star-like objects of a mysterious nature. However, the HST managed to detect several quasars, and went on to discover that they were all situated at the center of their host galaxies. Currently, most astronomers think that supermassive black holes, hiding in voracious secret within the hearts of galaxies, are the “engines” that power brilliant quasars.
Quasars appeared as point sources in early optical images, and they were almost indistinguishable from stars. However, there was one exception that suggested they were different–they had a weird, tattle-tale spectra. Using the HST, as well as infrared telescopes, the mysterious region encircling the quasar, has been observed in some instances by curious astronomers. The host galaxies are usually much too dim to be seen against the fierce glare of the brilliant quasar, unless astronomers use certain special techniques. Most quasars cannot be observed with small telescopes.
The luminosity of some quasars changes very quickly in the optical range of the electromagnetic spectra, and even more rapidly in the X-ray range. Because these changes occur so quickly, they can reveal an upper limit on the volume of a quasar, demonstrating that quasars are not much larger than our Solar System. Therefore, this indicates a huge energy density. The mechanism behind the alterations in brightness likely involves relativistic beaming of jets that are pointed almost directly toward Earth.
Before the HST was launched only a handful of black hole candidates had been observed. However, the severe limitations of ground based astronomy were such that substantial evidence for their real existence in nature could not be obtained. By their very definition, black holes evade detection because no light can escape from their powerful gravitational grip.
The good news is that astronomers can observe the effects of black holes on their surroundings. These include strong jets of electrons that travel vast distances, many thousands of light-years from the secretive centers of the galaxies.
The ill-fated matter tumbling towards a black hole, that is greedily awaiting its dinner, can also be observed screaming out brilliant light. If the speed of this infalling matter can be measured, it is possible to determine the black hole’s mass.
The larger the galaxy, the larger its resident supermassive black hole. It is generally thought that there must be some mechanism that links the birth of the galaxy to that of its black hole and vice versa. This discovery has profound implications for theories of galaxy formation, and it is an ongoing area of research in astronomy.
But why do most galaxies in our own cosmic neighborhood, including our Milky Way Galaxy, have dormant black holes that are not sucking in large quantities of matter at present?
Today many astronomers think that quasars, radio galaxies and the centers of so-called “active galaxies” are simply different views of more or less the same thing: a black hole with energetic jets streaming out from two sides. When the beam is aimed towards us we see the brilliant lighthouse of a quasar. However, when the orientation of the system is different we see it as an active galaxy or a radio galaxy. This unified model has obtained a great deal of support.
More than 200,000 quasars are currently known, and most of them were discovered by astronomers using data derived from the Sloan Digital Sky Survey (SDSS). SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope located at Apache Point, New Mexico in the U.S. It began gathering data in 2000.
Ancient Black Holes That Grew Up Too Fast
The trio of quasars, studied by the team of scientists at the Max Planck Institute for Astronomy, have approximately a billion times the mass of our Sun. All current theories explaining the way black holes grow propose that, in order to grow that massive, the black holes would have needed to accrete infalling matter, and shine brilliantly as quasars, for at least a hundred million years. But this trio of quasars seem to have been active for a much shorter period of time–less than 100,000 years. “This is a surprising result. We don’t understand how these young quasars could have grown to be the supermassive black holes that power them in such a short time,” Christina Eilers explained in a May 11, 2017 MPIA Press Release.
In order to determine the length of time that the trio of precocious quasars had been active, the astronomers studied the way they had influenced their environment–paying particular attention to the heated, mostly transparent “proximity zones” surrounding each quasar. “By simulating how the light from quasars ionizes and heats gas around them, we can predict how large the proximity zone of each quasar should be,” explained Dr. Frederick Davies in the MPIA Press Release. Dr. Davies is a postdoctoral researcher at MPIA who is an expert in the interaction between quasar light and intergalactic gas. Once the quasar has been ignited by infalling, unfortunate matter, these proximity zones grow very quickly. “Within a lifetime of 100,000 years, quasars should already have large proximity zones,” Dr. Davies added.
The surprise came when the astronomers discovered that the trio of quasars had very small proximity zones. This is because that indicates the active quasar phase cannot have started more than 100,000 years earlier. “No current theoretical models can explain the existence of these objects. The discovery of these young objects challenges the existing theories of black hole formation and will require new models to better understand how black holes and galaxies formed,” explained Dr. Joseph Hennawi in the May 11, 2017 MPIA Press Release. Dr. Hennawi leads the research group at MPIA that made this important discovery.
The astronomers have already made plans for their next steps, and have applied for telescope time to observe several additional candidates. The results, they hope, will constrain new theoretical models concerning the birth of the first supermassive black holes to haunt the observable Universe–and, by implication, help astronomers to understand the history of the giant supermassive black holes lurking in the secretive hearts of present-day galaxies like our own Milky Way.
Christina Eilers commented in the May 11, 2017 MPIA Press Release that “We would like to find more of these young quasars. While finding these three unusual quasars might have been a fluke, finding additional examples would imply that a significant fraction of the known quasar population is much younger than expected.”
The results of this study have been published in the May 2, 2017 edition of the Astrophysical Journal.
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.