Black holes are the most massive objects in the universe. Their gravitational pull is so strong that nothing can escape it — not even light. But according to a new NASA study, black holes may play a more complex role in galactic ‘ecosystems’. Specifically, a black hole was found to be contributing to the formation of a new star in its vicinity, offering tantalizing clues about how massive black holes develop in the first place.
A stellar nursery
Some ten years ago, Amy Reines, then a graduate student, discovered a black hole in a galaxy about 30 million light-years away from Earth, in the southern constellation Pyxis. She knew something was off right away, but it wasn’t until recently that new Hubble observations shed light on the situation.
“At only 30 million light-years away, Henize 2-10 is close enough that Hubble was able to capture both images and spectroscopic evidence of a black hole outflow very clearly. The additional surprise was that, rather than suppressing star formation, the outflow was triggering the birth of new stars,” said Zachary Schutte, Reines’ graduate student and lead author of the new study.
The galaxy, called Henize 2-10, is a so-called “starburst” galaxy — a galaxy where stars are being formed at a much higher rate than normal, around 1,000 times faster. The galaxy is also relatively small — a so-called dwarf galaxy — and has a black hole at its center, much like the Milky Way.
Researchers were already aware of an unusual cocoon of gas in the area, but Hubble managed to also image an outflow linked to the central black hole. Although the process is not fully understood, astronomers do believe that black holes (or at least some black holes) do have an outflow despite their massive gravity. In Henize 2-10, this outflow moves at about a million miles per hour, slamming into the gas cocoon — and as it turns out, newborn stars follow the path of the outflow.
In large galaxies, the opposite happens: material falling towards the black hole forms jets of plasma that don’t allow the formation of stars. But apparently, in the less-massive Henize 2-10, the outflow has just the right characteristics to precipitate new star formation. Previously, studies mostly focused on larger galaxies, where there is more observational evidence. Dwarf galaxies are still understudied, and it’s only thanks to Hubble that researchers were able to study this.
“Hubble’s amazing resolution clearly shows a corkscrew-like pattern in the velocities of the gas, which we can fit to the model of a precessing, or wobbling, outflow from a black hole. A supernova remnant would not have that pattern, and so it is effectively our smoking-gun proof that this is a black hole,” Reines said.
The role that black holes play in the universe is one of the biggest puzzles in astronomy, and the more data comes in, the more it’s starting to look like this is not a straightforward role, but rather a complex one. For instance, it was just recently demonstrated that researchers realized that most (if not all) galaxies have a black hole at their center. The more massive the galaxy, the more massive the central black hole — or possibly, the other way around, and the mass of the black hole is affecting the galaxy.
But we don’t really know how these central black holes (often called supermassive black holes) formed. Some researchers suspect they formed like “regular” black holes and somehow accumulated more and more mass; others believe they could only have formed in special conditions in the early stages of the universe; a further competing theory claims that the “seeds” of these black holes come from dense star clusters that collapse gravitationally. The black hole in Henize 2-10 could offer clues about these theories.
The black hole in the galaxy remained relatively small over cosmic time and did not accumulate a lot of material. This would suggest that it’s relatively unchanged since its formation, essentially offering a window into the early days of the universe.
“The era of the first black holes is not something that we have been able to see, so it really has become the big question: where did they come from? Dwarf galaxies may retain some memory of the black hole seeding scenario that has otherwise been lost to time and space,” Reines concludes.
By studying the motions of distant stars around the galactic centers, scientists showed that there’s a supermassive black hole at the heart of the Milky Way whose mass exceeds 4 million Suns. Continuing this line of research, which was awarded the 2020 Nobel Prize in Physics, astronomers affiliated with the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) have computed new images of the closest stars observed circling the supermassive black hole so far. The new images zoom in 20 times closer than what was previously possible.
Scientists have long-suspected the Milky Way harbors a massive black hole, like other similarly-sized galaxies in the universe. But proving it is another thing. After all, the gravitational anomaly could also be explained by tight clusters of neutron stars, for instance.
But all shroud of doubt lifted in the spring of 2002 when astrophysicists led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany used optical imaging showing a tiny speck of light — a star now known as SO-2 — that passed within 17 light-hours of the galactic center at an astonishing speed. That’s a minuscule distance at the cosmic scale, only three times the distance between Pluto and the sun. There’s only one object in the known universe that is compact enough and has enough mass to accelerate stars to such a high speed, and that’s a supermassive black hole.
Elsewhere, astronomer Andrea Ghez’s Galactic Center Group at UCLA used the 10-meter Keck Telescope in Hawaii to track the motion of S2, reporting in 2000 that the star’s path is curved, a telltale sign it was orbiting something super massive at the galactic center. The UCLA team later also found S2 orbits the galactic center, known as Sagittarius A*, very closely. Genzel, Ghez, and Roger Penrose shared the 2020 Nobel Prize in Physics “for the discovery of a supermassive compact object at the center of our galaxy.”
Research in studying the galactic center is an endless work in progress, though. Genzel’s latest work with the GRAVITY collaboration continues to expand an almost three-decade-long study of stars orbiting Sagittarius A*. GRAVITY is the second-generation VLTI instrument for precision narrow-angle astrometry and interferometric imaging. It brings the most advanced vision to the VLT: with its fiber-fed integrated optics, wavefront sensors, fringe tracker, beam stabilization, and a novel metrology concept, GRAVITY pushes the sensitivity and accuracy far beyond what is offered today. Using novel analysis techniques on GRAVITY data, the scientists obtained the deepest and sharpest images of the galactic center thus far.
In the process, the researchers made more precise measurements of previously identified stars as they approached the black hole. One such star is S29, which in May 2021 passed the galactic center at a distance of just 13 billion kilometers, equivalent to 90 times the distance from Earth to the Sun, at a speed of 8740 kilometers per second. No other star has been found to travel to this close or this fast around the supermassive black hole.
They also found a new, previously hidden star called S300, which demonstrated the power of the new method that combines the light of all four 8.2-meter telescopes of ESO’s Very Large Telescope located in Chile, using a technique known as interferometry. A machine-learning technique called Information Field Theory simulated how GRAVITY should see the new images of the stars around Sagittarius A*, which was then compared to actual GRAVITY observations.
“We have been building GRAVITY for a decade. How many things can go wrong when building such a complex machine? And, indeed, one of the main challenges really was that one needs to control & monitor the four telescopes of the VLT and the common interferometric laboratory to sufficient precision. Many, many control loops are running to keep things stable. And what perhaps is technically amazing: We use the 8m diameter telescopes, with domes as big as a tennis court, bundle the light, feed it into fibers that guide the light in channels with a diameter smaller than 1mm, and combine the light finally in a glass chip, that is only a few centimeters in size. The recorded wave patterns encode in a complex way the image of the sky – but with clever techniques, this can be recovered,” Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics and co-author of the new study told ZME Science.
The new observations also confirm that the paths of the stars in close proximity to Sagittarius A* are exactly those predicted by Einstein’s Theory of General Relativity. Using the new data, the researchers refined the mass of Sagittarius A*, now computed at 4.3 million times that of the Sun, as well as its distance, finding it is about 27,000 light-years away.
But despite the most precise measurements to date of Sagittarius A*, the galactic center remains largely mysterious. Previous work by scientists from the Event Horizon Telescope led to the first and now iconic image of the shadow of a black hole at the heart of galaxy M87. However, The Milky Way’s black hole is trickier to image because it is much more active. “I would not be surprised if also the new stars behave as chaotically as the ones we know at larger radii already. But we better find that out via observations,” Gillessen said.
Later this decade, GRAVITY will be upgraded in order to improve sensitivity and reveal fainter stars that perhaps lurk even closer to the black hole. ESO’s upcoming Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert, will further supercharge these efforts, allowing scientists to measure the velocity of these stars with the highest precision. In the meantime, even the tools currently at our disposal can deliver stunning results when in the right hands.
“Perhaps what is most stunning is how big a step forward it is. With interferometry we can see a factor 15 sharper than what was possible with single telescopes. That is really a huge improvement. Imagine, someone would increase your salary by that factor – it is simply huge. And we are very thrilled by the new opportunities. The Galactic Center continues to be very rich and exciting!” Gillessen said.
Black holes are cosmic bodies that pack an immense amount of mass into a surprisingly small space. Due to their extremely intense gravity, nothing can escape their grasp — not even light which defines the universe’s speed limit.
April 10th, 2019 marked a milestone in science history when the team at the Event Horizon Telescope revealed the first image of a supermassive black hole. As a result, these areas of space created when stars reach the end of their nuclear fuel burning and collapse creating massive gravitational wells, completed their transition from theory to reality.
This transition has been further solidified since with the revelation of a second, much clearer image of the supermassive black hole (SMBH) at the centre of the galaxy Messier 87 (M87). This second image revealing details such as the orientation of the magnetic fields that surround it and drive its powerful jets that extend for light-years.
The study of black holes could teach us much more than about these spacetime events and the environments that home them, however. Because cosmologists believe that most galaxies have an SMBH sat at their centre, greedily consuming material like a fat spider lurking at the centre of a cosmic web, learning more about these spacetime events can also teach us how galaxies themselves evolve.
The origin of black holes is one that runs in reverse to that of most astronomical objects. We didn’t discover some mysterious object in the distant cosmos and then began to theorise about it whilst making further observations.
Rather, black holes entered the scientific lexicon in a way that is more reminiscent of newly theorised particles in particle physics; emerging first from the solutions to complex mathematics. In the case of black holes, the solutions to the field equations employed by Einstein in his most important and revolutionary theory.
Just as a physical black hole forms from the collapse of a star, the theory of black holes emerged from the metaphorical collapse of the field equations that govern the geometrical theory of gravity; better known as general relativity.
One of the most common misconceptions about black holes arises from their intrinsic uniqueness and the fact that there really isn’t anything else like them in the Universe.
That’s Warped: Black Holes and Their Effect on Spacetime
General relativity introduced the idea that mass has an effect on spacetime, a concept fundamental to the idea that space and time are not passive stages upon which the events of the universe play out. Instead, those events shape that stage. As John Wheeler brilliantly and simply told us; when it comes to general relativity:
“Matter tells space how to curve. Space tells matter how to move.”
The most common analogy is for this warping of space is that of placing objects on a stretched rubber sheet. The larger the object the deeper the ‘dent’ and the more extreme the curvature it creates. In our analogy, a planet is a marble, a star an apple, and a black hole a cannonball.
Thus, considering this a black hole isn’t really ‘an object’ at all but, is actually better described as a spacetime event. When we say ‘black hole’ what we really mean is an area of space that is so ‘warped’ by a huge amount of mass condensed into a finite point that even light itself doesn’t have the necessary velocity to escape it.
This point at which light can no longer escape marks the first of two singularities that define black holes–points at which solutions of the equations of general relativity go to infinity.
The Event Horizon and the Central Singularity
The event horizon of a black hole is the point at which its escape velocity exceeds the speed of light in vacuum (c). This occurs at a radius called the Schwarzchild radius–named for astrophysicist Karl Schwarzschild, who developed a solution for Einstien’s field equations whilst serving on the Eastern Front in the First World War.
His solution to Einstein’s field equations–which would unsurprisingly become known as the Schwarzschild solution– described the spacetime geometry of an empty region of space. It had two interesting features — two singularities — one a coordinate singularity the other, a gravitational singularity. Both take on significance in the study of black holes.
Dealing with the coordinate singularity, or the Schwarzchild radius first.
The Schwarzchild radius (Rs) also takes on special meaning in cases where the radius of a body shrinks within this Schwarzschild radius (ie. Rs >r). When a body’s radius shrinks within this limit, it becomes a black hole.
All bodies have a Schwarzschild radius, but as you can see from the calculation below for a body like Earth, Rs falls well-within its radius.
That’s part of what makes black holes unique; their Schwartzchild radius is outside their physical radius because their mass is compressed into such a tiny space.
Because the outer edge of the event horizon is the last point at which light can escape it also marks the last point at which events can be seen by distant observers. Anything past this point can never be observed.
The reason the Schwarzschild radius is called a ‘coordinate singularity’ is that it can be removed with a clever choice of coordinate system. The second singularity can’t be dealt with in this way. This makes it the ‘true’ physical singularity of the black hole itself.
This is known as the gravitational singularity and is found at the centre of the black hole (r=0). This is the end-point for every particle that falls into a black hole. It’s also the point the Einstein field equations break down… maybe even all the laws of physics themselves.
The fact that the escape velocity of the event horizon exceeds the speed of light means that no physical signal could ever carry information from the central singularity to distant observers. We are forever sealed off from this aspect of black holes, which will therefore forever remain in the domain of theory.
How to Make a Black Hole
We’ve already seen that for a body with the mass of Earth to become a black hole, its diameter would have to shrink to less than 2cm. This is obviously something that just isn’t possible. In fact, not even our Sun has enough mass to end its life as a black hole. Only stars with around three times the mass of the Sun are massive enough to end their lives in this way.
But why is that the case?
It won’t surprise you to learn that for an astronomical body to become a black hole it must meet and exceed a series of limits. These limits are created by outward forces that are resisting against the inward force that leads to gravitational collapse.
For planets and other bodies with relatively small masses, the electromagnetic repulsion between atoms is strong enough to grant them stability against total gravitational collapse. For large stars the situation is different.
During the main life cycle of stars–the period of the fusion of hydrogen atoms to helium atoms–the primary protection against gravitational collapse is the outward thermal and radiation pressures that are generated by these nuclear processes. That means that the first wave of gravitational collapse occurs when a star’s hydrogen fuel is exhausted and inward pressure can no longer be resisted.
Should a star have enough mass, this collapse forces together atoms in the nucleus enough to reignite nuclear fusion— with helium atoms now fusing to create heavier elements. When this helium is exhausted, the process happens again, with the collapse again stalling if there is enough pressure to trigger the fusion of heavier elements still.
Stars like the Sun will eventually reach the point where their mass is no longer sufficient to kick start the nuclear burning of increasingly heavier elements. But if it isn’t nuclear fusion that is generating the outward forces that prevent complete collapse, what is preventing these lower-mass stars from becoming black holes?
Placing Limits on Gravitational Collapse
Lower-mass stars like the Sun will end their lives as white dwarf stars with a black hole form out of reach. The mechanism protecting these white dwarfs against complete collapse is a quantum mechanical phenomenon calleddegeneracy.
This ‘degeneracy pressure’ is a factor of the Pauli exclusion principle, which states that certain particles– known as fermions, which include electrons, protons, and neutrons– are forbidden from occupying the same ‘quantum states.’ This means that they resist being tightly crammed together.
This theory and the limitation it introduced led Indian-American astrophysicist Subrahmanyan Chandrasekhar to question if there was an upper cap at which this protection against gravitational collapse would fail.
Chandrasekhar –awarded the 1983 Nobel Prize in physics for his work concerning stellar evolution– proposed in 1931 that above 1.4 solar masses, a white dwarf would no longer be protected from gravitational collapse by degeneracy pressure. Past this limit — termed the Chandrasekhar limit — gravity overwhelms the Pauli exclusion principle and gravitational collapse can continue.
But there is another limit that prevents stars of even this greater mass from creating black holes.
Thanks to the 1932 discovery of neutrons— the neutral partner of protons in atomic nuclei — Russian theoretical physicist Lev Landau began to ponder the possible existence of neutron stars. The outer part of these stars would contain neutron-rich nuclei, whilst the inner sections would be formed from a ‘quantum fluid’ comprised of mostly neutrons
These neutron stars would also be protected against gravitational collapse by degeneracy pressure — this time provided by this neutron fluid. In addition to this, the greater mass of the neutron in comparison to the electron would allow neutron stars to reach a greater density before undergoing collapse.
By 1939, Robert Oppenheimer had calculated that the mass-limit for neutron stars would be roughly 3 times the mass of the Sun.
To put this into perspective, a white dwarf with the mass of the Sun would be expected to have a millionth of our star’s volume — giving it a radius of 5000km, roughly that of the Earth. A neutron star of a similar mass though would have a radius of about 20km — roughly the size of a city.
Above the Oppenheimer-Volkoff limit, gravitational collapse begins again. This time no limits exist between this collapse and the creation of the densest possible state in which matter can exist. The state found at the central singularity of a black hole.
We’ve covered the creation of black holes and the hurdles that stand in the way of the formation of such areas of spacetime, but theory isn’t quite ready to hand black holes over to practical observations just yet. The field equations of general relativity can also be useful in the categorisation of black holes.
The four types of black holes
Categorising black holes is actually fairly straight-forward thanks to the fact that they possess very few independent qualities. John Wheeler had a colourful way of describing this lack of characteristics. The physicist once commented that black holes ‘have no hair,’ meaning that outside a few characteristics they are essentially indistinguishable. This comment became immortalised as the no-hair theorem of black holes.
Black holes have only three independent measurable properties — mass, angular momentum and electric charge. All black holes must have mass, so this means there are only four different types of a black hole based on these qualities. Each is defined by the metric or the function used to describe it.
This means that black holes can be quite easily catagorised by the properties they possess as seen below.
This isn’t the most common or most suitable method of categorising black holes, however. As mass is the only property that is common to all black holes, the most straight-forward and natural way of listing them is by their mass. These mass categories are imperfectly defined and so far black holes in some of the categories–most notably intermediate black holes– remain undetected.
Cosmologists believe that the majority of black holes are rotating and non-charged Kerr black holes. And the study of these spacetime events reveals a phenomenon that perfectly exemplifies their power and influence on spacetime.
The Anatomy of a Kerr Black Hole
The mathematics of the Kerr metric used to describe non-charged rotating black holes reveals that as they rotate, the very fabric of spacetime that surrounds them is dragged along in the direction of the rotation.
The powerful phenomenon is known as ‘frame-dragging’ or the Lense-Thirring effect and leads to the violent churning environments that surround Kerr black holes. Recent research has revealed that this frame-dragging could be responsible for the breaking and reconnecting of magnetic field lines that in-turn, launch powerful astrophysical jets into the cosmos.
The static limit of a Kerr black hole also has an interesting physical significance. This is the point at which light–or any particle for that matter– is no-longer free to travel in any direction. Though not a light-trapping surface like the event horizon, the static limit pulls light in the direction of rotation of the black hole. Thus, light can still escape the static limit but only in a specific direction.
British theoretical physicist and 2020 Nobel Laureate Sir Roger Penrose also suggested that the static limit could be responsible for a process that could cause black holes to ‘leak’ energy into the surrounding Universe. Should a particle decay into a particle and its corresponding anti-particle at the edge of the static limit it would be possible for the latter to fall into the black hole, whilst its counterpart is launched into the surrounding Universe.
This has the net effect of reducing the black hole’s mass whilst increasing the mass content of the wider Universe.
We’ve seen what happens to light at the edge of a black hole and explored the fate of particles that fall within a Kerr black hole’s static limit, but what would happen to an astronaut that strayed too close to the edge of such a spacetime event?
Death by Spaghettification
Of course, any astronaut falling into a black hole would be completely crushed upon reaching its central gravitational singularity, but the journey may spell doom even before this point has been reached. This is thanks to the tidal forces generated by the black hole’s immense gravitational influence.
As the astronaut’s centre of mass falls towards the black hole, the object’s effect on spacetime around it causes their head and feet to arrive at significantly different times. The difference in the gravitational force at the astronaut’s head and feet gives rise to such a huge tidal force that means their body would be simultaneously compressed at the sides and stretched out.
Physicists refer to this process as spaghettification. A witty name for a pretty horrible way to die. Fortunately, we haven’t yet lost any astronauts to this bizarre demise, but astronomers have been able to watch stars meet the same fate.
For a stellar-mass black hole, spaghettification would occur not just before our astronaut reaches the central singularity, but also well before they even hit the event horizon. For a black hole 40 times the mass of our Sun — spaghettification would occur at about 1,000 km out from the event horizon, which is, itself, 120 km from the central gravitational singularity.
As well as developing the Oppenheimer-Volkoff limit, Oppenheimer also used general relativity to describe how a total gravitational collapse should appear to a distant observer. They would consider the collapse to take an infinitely long time, the process appearing to slow and freeze as the star’s surface shrinks towards the Schwarzschild radius.
An astronaut falling into a black hole would be immortalized in a similar way to a distant observer, though they themselves–could they have survived spaghettification– they would notice nothing. The passing of Rs would just seem a natural part of the fall to them despite it marking the point of no return.
Much More to Learn…
After emerging from the mathematics of general relativity at the earlier stages of the 20th Century, black holes have developed from a theoretical curiosity to the status of scientific reality. In the process, they have indelibly worked their way into our culture and lexicon.
Perhaps the most exciting thing about black holes is that there is so much we don’t yet know about them. As a striking example of that, almost all the information listed above resulted just from theory and the interrogation of the maths of Einstein’s field equations.
Unlocking the secrets held by black holes could, in turn, reveal how galaxies evolve and how the Universe itself has changed since its early epochs.
Sources and Further Reading
Relativity, Gravitation and Cosmology, Robert J. Lambourne, Cambridge Press, .
Relativity, Gravitation and Cosmology: A basic introduction, Ta-Pei Cheng, Oxford University Press, .
With assistance from the ESO’s Very Large Telescope (VLT), astronomers have discovered the most distant radio emission ever recorded. The source is a quasar so distant that its light has been travelling 13 billion years to reach us. That means that it existed when the Universe was just 780 or so million years old.
The object–named P172+18–is what astronomers term a ‘radio loud’ quasar, shining powerfully in the radio-frequency region of the electromagnetic spectrum, extremely bright due to the powerful jets emitted from its axis. Radio loud quasars are fairly rare with only 10% of discovered quasars fitting this description.
This makes the team’s finding even more extraordinary as even though more distant quasars have been found, it marks the first time that researchers have been able to identify the tell-tale signs of powerful radio-bright jets at such incredible cosmic distances.
Excitingly, the team at the centre of this finding believe that this is just the tip of the iceberg with regards to radio-loud quasars, with many more yet to be discovered. Possibly even some at much greater distances.
The team’s discovery is discussed in a paper published in the latest edition of The Astrophysical Journal.
Quasars: Powered By Black Holes
Quasars are objects that lie at the centre of galaxies, powered by supermassive black hole ‘engines.’ The black hole at the heart of P172+18 is a doozy. The team estimate it is around 300 million times the mass of the Sun. As impressive as that is, perhaps more staggering is the rate at which this supermassive black hole is consuming gas and dust.
“The black hole is eating up matter very rapidly, growing in mass at one of the highest rates ever observed,” says Chiara Mazzucchelli, co-leader of the project and an astronomer based at ESO, Chile. “I find it very exciting to discover ‘new’ black holes for the first time, and to provide one more building block to understand the primordial Universe, where we come from, and ultimately ourselves.”
The team believes that the rapid rate of gas consumption displayed by the supermassive black hole and its burgeoning growth are both intrinsically linked to the emission of the radio bright jets they detected. The jets could be disturbing gas in an accretion disc around the black hole, causing it to fall into the central black hole at an accelerated rate.
If this proves to be the case, the study of radio-loud quasars could be of vital importance in the future investigation of the growth of black holes in the infant Universe. There is currently some confusion as to how supermassive black holes could have grown to tremendous sizes over a relatively short-period in cosmic terms, thus a mechanism that accounts for rapid growth is a boon to cosmologists fearing that models of cosmic evolution could need fundamental revision.
Very Loud and Very Far Away
P172+18 was first spotted as a radio source in data gathered by t the Magellan Telescope at Las Campanas Observatory in Chile. Mazzucchelli and team co-leader Eduardo Bañados of the Max Planck Institute for Astronomy, Germany, then assessed the data and quickly concluded that the radio source represented jets produced by a distant radio-loud quasar.
“As soon as we got the data, we inspected it by eye, and we knew immediately that we had discovered the most distant radio-loud quasar known so far,” says Bañados.
Because P172+18 was only observed for a brief period, it was necessary for the duo to follow up the observations with other telescopes. They were able to do this with the use of the X-Shooter instrument associated with the VLT, based in the Atacama Desert, Chile, as well as the National Radio Astronomy Observatory’s Very Large Array (VLA) in New Mexico, and the Keck Telescope located near the summit of Mauna Kea, Hawaii.
These follow-up observations allowed the team to ascertain a wealth of details about the quasar and the supermassive black hole powering it, including its mass and the rapid rate at which it is consuming gas and surrounding matter.
P172+18 may currently hold the record for most distant radio-loud quasar, but it is not a distinction that Mazzucchelli and Bañados think it will hang on to for long. The duo believes that many more radio-loud quasars are lurking in the Universe waiting to be discovered and that undoubtedly, some of these will exist at greater distances than 13 billion light-years.
Whilst these may be a challenge to spot currently, the ESO’s forthcoming Extremely Large Telescope (ELT), currently under construction in Northern Chile, should be powerful enough to handle such observations.
“This discovery makes me optimistic and I believe — and hope — that the distance record will be broken soon,” concludes Bañados.
The core of a galaxy is typically brightly lit due to highly luminous objects known as quasars — the brightest in the universe actually — that are powered by gas spiraling at high velocity from an extremely large black hole. One such bright source was recently discovered by astronomers over 13 billion light-years away. The quasar in question is thus associated with the farthest and, at the same time, the oldest supermassive black hole encountered so far.
The quasar dubbed J0313-1806 is of great significance for scientists since it formed just 670 million years after the universe as we know it came into existence. As such, the mysterious cosmic object may help shed light on how the ancient universe functioned, setting the stage for its evolution in its current configuration. Previously, the oldest black hole found by scientists was 690 million years old.
But how can black holes be so luminous — aren’t they supposed to suck up everything in their vicinity, even light? That’s of course still true, but when a black hole reaches gargantuan proportions millions to billions of times more massive than the sun, its gravity can create a whirlwind of gas and dust. As matter is funneled into the black hole, it forms a trail of debris shaped like a disk around the black hole. This debris revolves at dizzying speeds and, hence, high energy, which is expelled into the universe. Here on Earth, we can see these eruptions of electromagnetic radiation in the visible spectrum.
And this radiation is extremely intense. The authors of the new study — who studied the quasar using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the world’s largest radio telescope, and two observatories on Mauna Kea in Hawaii — found that J0313-1806 is about 1,000 times brighter than all of the Milky Way galaxy.
But as it often has it when speaking of black holes, for every new thing we learn about them, there are also new questions that scientists can’t answer yet.
Calculations performed by the team of researchers, which included lead author Feige Wang, a Hubble Fellow at the University of Arizona’s Steward Observatory, suggest that the quasar’s black hole has a mass of around 1.6 billion times that of the sun — far too great for such a short period of growth time. Where did all that mass come from considering the quasar is so young?
Some of the available ‘food’ for black holes in the early universe would have been first-generation massive stars largely made of hydrogen and lacking in heavy elements that make up later stars. But even if the black hole formed as early as 100 million years after the Big Bang and grew as quickly as possible, it still would have had to have at least 10,000 solar masses from the very beginning. Things don’t add up, which tells us that astronomers are still missing some pieces of the puzzle. But the authors of the new study have some ideas.
“This tells you that no matter what you do, the seed of this black hole must have formed by a different mechanism,” said co-author Xiaohui Fan, Regents Professor and associate head of the University of Arizona Department of Astronomy. “In this case, one that involves vast quantities of primordial, cold hydrogen gas directly collapsing into a seed black hole.”
The monster black hole at J0313-1806 is still growing at a rapid pace, gobbling up the equivalent of 25 suns every day. How exactly such a supermassive black hole came into existence could be answered with data provided by the upcoming and much-anticipated James Webb Space Telescope, slated to launch on October 31.
Astronomers surveying the heart of the giant galaxy cluster Abell 2261 were expecting to find a hefty supermassive black hole, with an appetite for matter and energy that matched the enormous scale of its cosmic home. But although the supermassive black hole should have had a mass between 3 to 100 billion times that of the sun — very conspicuous to most observations — there was no object to be found. NASA scientists don’t know what to make of the missing black hole and its elusive nature may be yet another reminder that we still know very little about these mysterious objects.
Just like stars cluster around the core of a galaxy, so do galaxies group together, bound by the gravity of an extremely dense core. Most known galaxies, including the Milky Way, have a supermassive black hole at their core that keeps their stars together. Similarly, clusters of hundreds or thousands of galaxies have a core, with an even larger supermassive black hole bounding all those galaxies in place. The galaxy nearest this core is known as the brightest cluster galaxy (BCG) — this is where the cluster’s most important supermassive black hole should be found.
Abell 2261’s BCG is a staggering one million light-years across, making it about 10 times larger than the Milky Way. What’s more, it has a huge core 10,000 light-years across, the largest galactic core ever found.
But, defying all expectations, a team of astronomers led by Kayhan Gultekin from the University of Michigan couldn’t find any supermassive black hole in the galaxy in the middle of Abell 2261, located about 2.7 billion light-years from Earth.
The astronomers employed the most sophisticated techniques and searched the sky using NASA’s Chandra X-Ray Observatory and the Hubble Space Telescope, however they were quite surprised when they came out empty-handed. After all, an object that could weigh as much as 100 billion times the mass of the sun shouldn’t be able to hide easily. For comparison, the Milky Way’s supermassive black hole has 4 million solar masses.
As a black hole devours matter, some of the material becomes superheated at it falls towards the black hole, producing X-rays in the process. However, the scientists did not detect any such source. This is the second time that researchers have failed to find the supermassive black hole at the center of Abell 2261’s large central galaxy, after previous attempts that employed Chandra data obtained in 1999 and 2004. So perhaps there really isn’t any black hole — at least not where scientists have been expecting.
It’s possible the black hole was ejected from the host galaxy’s center as a result of two galaxies merging to form the observed one. When two black holes merge, they produce gravitational waves that may be stronger in one direction than another, so the newly formed supermassive black hole might have been ejected from the center of the galaxy in the opposite direction. Scientists call this a recoiling black hole.
But that’s all in theory. We don’t have any evidence that supermassive black holes can merge, let alone evidence of a black hole recoiling. To date, researchers have only confirmed the mergers of much smaller black holes.
Alternatively, the black hole may not be active enough to produce noticeable amounts of X-rays to show up in Chandra observations, although that sounds a bit implausible given the expected scale. Finally, the black hole may simply not exist.
These hypotheses may be verified once NASA’s upcoming James Webb Telescope comes into operation.
The findings are due to be published in the journal of the American Astronomical Society. In the meantime, it is available online on the pre-print server arXiv.
An international team of researchers has used telescopes from around the world — including instruments operated by the European Southern Observatory (ESO) — to glimpse a blast of light emitted by a star as it is torn apart by the tidal forces of a supermassive black hole.
The event — technically known as a ‘tidal disruption event’ (TDE) — occurred 215-million light-years from Earth, but despite this intimidating sounding distance, this is the closest to our planet such a flare has ever been captured. This, and the fact the astronomers spotted the event early, means the team was able to study the phenomena in unprecedented detail, in turn uncovering some surprises in this violent and powerful process.
The astronomers directed the ESO’s Very Large Telescope (VLT), based in the Atacama desert, Chile, and other instruments at a blast of light that first occurred last year. They studied the flare, located in AT2019qiz in a spiral galaxy in the constellation of Eridanus, for six months as it grew in luminosity and then faded. Their findings are published today in the Monthly Notices of the Royal Astronomical Society.
“My research focuses on close encounters between stars and supermassive black holes in the centres of galaxies. Gravity very close to a black hole is so strong that a star cannot survive, and instead gets ripped apart into thin streams of gas,” Thomas Wevers, co-author of the study and an ESO Fellow in Santiago, Chile, tells ZME Science. “This process is called a tidal disruption event, or sometimes ‘spaghettification’.
“If not for such tidal disruption events, we would not be able to see these black holes. Hence, they provide a unique opportunity to study the properties of these ‘hidden’ black holes in detail.”
Thomas Wevers, ESO Fellow
Catching the Start of the Movie
Wevers, who was part of the Institute of Astronomy, University of Cambridge, UK, as the study was being conducted, explains that it can take several weeks — or even months — to identify these spaghettification events with any certainty. Such an identification also takes all the telescopes and observational power that can be mustered. This can often cause a delay that results in astronomers missing the early stages of the process.
“It’s like watching a movie but starting 30 minutes in, a lot of information is lost if you can’t watch from the very beginning, and while you might be able to reconstruct roughly what has happened, you can never be completely sure,” the researcher explains. But, that wasn’t the case with this new event.
To stick to the analogy; this time the team had their popcorn and drink and were in their seats before the trailers started rolling.
“In this new event, we were lucky enough to identify and hence observe it very quickly, which has allowed us to see and understand what happens in the early phases in great detail.”
Thomas Wevers, ESO Fellow
Spotting spaghettification events is not just difficult due to timing issues, though. Such events are fairly rare, with only 100 candidates identified thus far, and are often obscured by a curtain of dust and debris. When a black hole devours a star, a jet of material is launched outwards that can further obscure the view of astronomers. The prompt viewing of this event allowed that jet to be seen as it progressed.
“The difficulty comes first from picking out these rare events in among all the more common things changing in the night sky: variable stars and supernova explosions,” Matt Nicholl, a lecturer and Royal Astronomical Society research fellow at the University of Birmingham, UK, and the lead author of the study tells ZME Science. “A second difficulty comes from the events themselves: they were predicted to look about 100 times hotter than the flare that we observed. Our data show that this is because of all the outflowing debris launched from the black hole: this absorbs the heat and cools down as it expands.”
Spaghettification: Delicious and Dangerous
The spaghettification process is one of the most fascinating aspects of black hole physics. It arises from the massive change in gravitational forces experienced by a body as it approaches a black hole.
“A star is essentially a giant ball of hot, self-gravitating gas, which is why it is roughly spherical in shape. When the star approaches the black hole, gravity acts in a preferential direction, so the star gets squeezed in one direction but stretched in the perpendicular direction,” Wevers says. “You can compare it to a balloon: when you squeeze it between your hands, it elongates in the direction parallel to your hands. Because the gravity is so extreme, the result is that the star essentially gets squeezed into a very long and thin spaghetti strand — hence the name spaghettification.”
Nicholl continues, explaining what happens next to this stellar spaghetti strand: “Eventually, it wraps all the way around and collides with itself, and that’s when we start to see the light show as the material heats up before either falling into the black hole or being flung back into space.
“The distance at which the star encountered the supermassive black hole was around the same distance between the Earth and Sun — this shows how incredibly strong the gravitational pull of the black hole must be to be able to tear the star apart from that distance.”
“If you picture the Sun being torn into a thin stream and rushing towards us, that’s roughly what the black hole saw!”
Matt Nicholl, Royal Astronomical Society research fellow.
Suprises and Future Developments
The observations made by the astronomers have allowed them to study the dynamics of a star undergoing the spaghettification process in detail, something that hasn’t been possible before. And as is to be expected with such a first, the study yielded some surprises for the team.
“The biggest surprise with this event was how rapidly the light brightened and faded,” Nicholl tells ZME. “It took about a month from the encounter for the flare to reach its peak brightness, which is one of the fastest we’ve ever seen.”
The researcher continues to explain that faster events are harder to find, so it suggests that there might be a whole population of short-lived flares that have been escaping astronomers’ attention. “Our research may have solved a major and long-standing mystery of why these events are 100 times colder than expected — in this event, it was the outflowing gas that allowed it to cool down.”
Confirming this idea means that the team must now seek scarce telescope time to investigate more of these events to see if this characteristic is unique to the AT2019qiz flare, or if it is a common feature of such events. “Because we studied only one event, it is still unclear whether our results apply universally to all such tidal disruption events. So we need to repeat our experiment multiple times,” Wevers says. “Unfortunately, we are at the whims of nature and our ability to spot new TDEs. When we do, we will need to confirm the picture we have put forward or perhaps adapt it if we find different behaviour.”
Wevers concludes by highlighting the unique position he, Nicholl, and their team find themselves in by studying such rare and difficult to observe events and the objects that lie behind them. “We aren’t yet in the phase where we think we have mapped all the behaviour that occurs following these cataclysmic events, so while each new TDE helps us to answer outstanding questions, at the same time it also raises new questions.
“We find ourselves continually in a catch-22 like situation, which in this case is a good thing as it propels our research forward!” exclaims Wevers. “I find it pretty amazing that we can study gargantuan black holes, weighing millions or even billions of times the mass of our sun, and which are located hundreds of millions of light-years away, in such detail with our telescopes.”
Original research: Nicholl. M., Wevers. T., Oates. S. R., et al, ‘An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz,’ Monthly Notices of the Royal Astronomical Society, .
Astronomers have discovered a tremendous cosmic web in the early Universe. Trapped within its threads are six galaxies feeding gas to a central supermassive black hole.
Astronomers have made a startling discovery in the early Universe: six galaxies suspended in the cosmic web of a supermassive black hole. The finding represents the first time such a grouping has been found when the Universe was young — just under a billion years after the ‘Big Bang.’
The cosmic web with its suspended galaxies seems to conform to the theory that supermassive black holes grew to monstrous sizes thanks to the fact that they sat at the centre of web-like structures with gas and dust to feed them.
“The title of our article — ‘Web of the Giant’ — may suggest the idea of the supermassive black hole as a giant black spider at the centre of the web, with that web providing both the trap and the path to carry the material that feeds the giant at the centre,” Marco Mignoli, an astronomer at the National Institute for Astrophysics (INAF) in Bologna, Italy, tells ZME Science. “The importance of our work is that we are the first group to discover the galaxies that inhabit the web.”
The new observation of these six faint galaxies trapped in a web of filament ‘threads’ comprised of hot gas, stars, and galaxies surrounding a supermassive black hole, was made with the aid of the ESO’s Very Large Telescope (VLT) and is described in a paper published in the journal Astronomy and Astrophysics.
As is only fitting for a supermassive black hole ‘spider’, the web in which it sits is of tremendous size — 300 times that of the Milky Way. “From this filamentous structure, the giant black hole is probably accumulating material that has allowed it to grow extremely fast, reaching one billion solar masses in less than a billion years,” Mignoli, author of the paper, adds. “The galaxies stand and grow where the filaments cross, and streams of gas — available to fuel both the galaxies and the central supermassive black hole — can flow along the filaments.”
Supermassive Black Hole Feeding in the Early Universe
The light from this cosmic web has traveled to us from a time when the Universe was just 0.9 billion years old. This represents not just a time at which the first generation of black holes have formed from collapsing stars, but also the point where the faster-growing of these black holes have grown to truly monstrous sizes.
The question of how supermassive black holes managed to grow so rapidly has puzzled scientists for decades, with researchers unable to detect exactly how these black holes could obtain so much ‘black hole fuel’ so quickly. The findings seem to provide an answer, suggesting that the cosmic web and the galaxies within it contain enough gas to quickly grow the central black hole into a supermassive giant.
“[The study] provides confirmation of several theories, that these primordial supermassive black holes are found at the center of immense filamentous structures composed by hot gas and by galaxies that are actively forming stars,” Mignoli says. “Such structures — ‘cosmic webs’ — can provide the necessary material for the central black hole to grow extremely fast.”
Whilst this isn’t the first time astronomers have spotted a ‘cosmic web’, it is the first time its been occupied by a supermassive black hole ‘spider’ at its heart.
“Similar, early large scale structures have already been found, but none with a supermassive black hole at their centre,” Roberto Gilli, an astronomer at INAF in Bologna and co-author of the study, tells ZME Science. “Our work has placed an important piece in the largely incomplete puzzle that is the formation and growth of such extreme, yet relatively abundant objects so quickly after the Big Bang.”
What the researchers can’t be so sure of is how these black holes initially formed, the process by which they are ‘fed’, or how the cosmic web itself developed. “We have no observational evidence of from which seeds these giant black holes are grown,” Mignoli explains. “The structures are too far away, the gas flows too faint to be detected. And also from a theoretical point of view, there are problems that are too difficult to solve.”
One possibility is that cosmic webs such as that discovered by the team formed as a result of the gravitational influence of dark matter haloes. These bunches of mysterious substance — which makes up 90% of the matter in the known Universe — could have drawn together tremendous amounts of gas in the early Universe. From there, the gas and dark matter may have formed the matrix of a cosmic web.
Mignoli explains that one of the most intriguing lingering questions is what process allows material to be transported from an intergalactic scale to the size of a black hole’s accretion disc — in the order of parsecs.
Gilli offers some suggestions regarding this feeding process, albeit ones that are currently unsupported by observations: “According to theory, dense environments are a necessary but not sufficient condition,” Gilli explains. “[The feeding mechanism] could be related to gas availability in these dense regions: large reservoirs mean that there is enough gas to fuel the BH and grow fast. Some theories propose that direct gas streams through the web can fall directly into the black holes and grow them.”
Gilli also adds that another way by which supermassive black holes could gather tremendous mass is via galactic mergers. And, the researcher adds, the cosmic web could play a role in this process too. “Another way this web can aid black hole growth is through galaxy mergers: within these dense, filamentary environments, mergers of gas-rich galaxies are more frequent, and mergers normally destabilize gas within galaxies and allow it to fall within the black holes at their centres.”
Spider Hunting: Searching the Universe for more ‘Occupied’ Cosmic Webs
The galaxies observed by the team are some of the faintest ever studied by astronomers, and required employing the tremendous power of the VLT — located at the ESO’s Paranal Observatory in the Atacama Desert, Chile — for several hours. Thus, with the aid of the VLT’s MUSE and FORS2 instruments, the team was able to confirm the six galaxies were linked to a central supermassive black hole.
“Early supermassive black holes are among the most challenging systems in extragalactic astrophysics,” Gilli explains. “We designed this experiment more than 8 years ago in the hope of confirming theory expectations. Observations of such systems are painful and only by cumulating several years of effort we could finally confirm the existence of such a structure.”#
The aim now is to find more examples of such structures, which Gilli believes should be fairly common in the early Universe. Perhaps then we can start to answer the questions that surround the formation and evolution of such events.
“We’d like to confirm more structure like this and also to discover populations of SMBHs in the early Universe that should exist but are still missing from our census,” Gilli says. “There are billion-solar-mass black holes at galaxies’ centres and we still do not know where they come from. And there is even more mystery surrounding such systems in the early Universe.”
Mignoli. M., Gilli. R., Decarli. R., et al, , ‘Web of the giant: Spectroscopic confirmation of a large-scale structure around the z=6.31 quasar SDSS J1030+0524,’ Astronomy and Astrophysics.
Ever want to make a literal dent in space? Apparently a supermassive black hole in the Ophiuchus galaxy cluster did when it erupted and caused the largest known explosion since the Big Bang. The black hole punched a dent the size of 15 Milky Ways in the surrounding space.
Galaxy clusters like Ophiuchus are some of the largest objects in the universe and contain thousands of individual galaxies, dark matter, and hot gas. At the heart of the Ophiuchus cluster is a giant galaxy that contains a black hole with a mass equivalent to around 10 million suns.
Professor Melanie Johnston-Hollitt, from the Curtin University node of the International Centre for Radio Astronomy Research, said the event was extraordinarily energetic. “We’ve seen outbursts in the centres of galaxies before but this one is really, really massive,” she said. “And we don’t know why it’s so big.”
The explosion — captured using four telescopes; NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton, the Murchison Widefield Array (MWA) in Western Australia and the Giant Metrewave Radio Telescope (GMRT) in India — occurred 390 million light-years from Earth. It was so powerful that it punched a cavity in the cluster plasma, the super-hot gas surrounding the black hole. The find was made with Phase 1 of the MWA, when the telescope had 2048 antennas pointed towards the sky.
Scientists dismiss the idea that it could be caused by an energetic outburst, as it was too large. The explosion was initially recorded by the Chandra Observatory in 2016, however, the results were dismissed as astronomers believed a cavity of that magnitude was impossible.
Lead author of the study Dr. Simona Giacintucci, from the Naval Research Laboratory in the United States, said the blast was similar to the 1980 eruption of Mount St. Helens, which ripped the top off the mountain. “The difference is that you could fit 15 Milky Way galaxies in a row into the crater this eruption punched into the cluster’s hot gas,” she said.
Professor Johnston-Hollitt, who is the director of the MWA and an expert in galaxy clusters, says that this finding underscores the importance of studying the universe at different wavelengths. She also likened the finding to discovering the first dinosaur bones.
“It’s a bit like archaeology,” she says. “We’ve been given the tools to dig deeper with low frequency radio telescopes so we should be able to find more outbursts like this now….We’re soon going to be gathering (MWA) observations with 4096 antennas, which should be ten times more sensitive. I think that’s pretty exciting.”
Dwarf galaxies have traditionally been considered too small to host massive black holes, but new research emerging from Montanna State University (MSU) has revealed dozens of examples. The research, published in the Astrophysical Journal has delivered another surprise, these black holes aren’t located where scientists usually expect to find them.
“All of the black holes I had found before were in the centres of galaxies,” says Amy Reines, an assistant professor in the Department of Physics in the College of Letters and Science. “These were roaming around the outskirts. I was blown away when I saw this.”
Reines and her team searched 111 dwarf galaxies within a radius of a billion-light-years of Earth using the National Science Foundation’s Karl G. Jansky Very Large Array at the National Radio Astronomy Observatory, Albuquerque, New Mexico. During the course of their search, they identified 13 galaxies that very probably host black holes, the majority of which were not centralised.
Reines is also a researcher in the MSU’s eXtreme Gravity Institute, which unites astronomers and physicists in order to study phenomena in which the gravitational influence is so powerful that it blurs the separation of space and time. This includes events and objects such as neutron stars, black holes, mergers and collisions between the two and even, the initial extreme period of rapid expansion of the universe — the big bang.
The researcher explains that whilst stellar-mass black holes — those with a mass of up to 10 times that of our Sun — form as large stars undergo gravitational collapse, we are, thus far, uncertain how supermassive black holes form. This class of black hole which can have masses of up to billions of times that of the Sun is most commonly found in the centre of galaxies.
This is certainly the case with our galaxy, the Milky Way, which hosts the supermassive black hole Sagittarius A* (SgrA*) at its centre. Dwarf galaxies are smaller than spiral galaxies like the Milky Way, containing a few billion stars rather than 100–400 billion as spiral galaxies tend to.
The results collected by Reines confirm computer simulations generated by Jillian Bellovary, assistant professor at Queensborough Community College, New York and Research Associate at the American Museum of Natural History.
How black holes get lost
Bellovary’s computer simulations suggested that black holes could be disturbed from the centre of dwarf galaxies by interactions they undergo as they travel through space. This result coupled with Reines’ study have the potential to change the way we look for black holes in dwarf galaxies going forward. This change in thinking could also impact theories of how both dwarf galaxies and supermassive black holes form.
“We need to expand searches to target the whole galaxy, not just the nuclei where we previously expected black holes to be,” Reines adds.
No stranger for the search for black holes, Reines has been hunting these events for a decade, ever since she was a graduate student at the University of Virginia. Whilst she initially focused on star formation in dwarf galaxies, her research led her to something else that captured her interest: a massive black hole “in a little dwarf galaxy where it wasn’t supposed to be.”
The little dwarf galaxy she refers to is Heinze 2–10, located 30-million-light-years from Earth, which had previously been believed too small to host a massive black hole. “Conventional wisdom told us that all massive galaxies with a spheroidal component have a massive black hole and little dwarf galaxies didn’t,” Reines explains, adding that when she discovered such a relationship it was a “eureka” moment. After publishing these findings in the journal Nature she continued searching for further black holes in dwarf galaxies. “Once I started looking for these things on purpose, I started finding a whole bunch,” Reines says.
Changing her tactics by shifting from visual data from radio signals, Reines uncovered over 100 possible black holes in her first search of a sample that included 40,000 dwarf galaxies. In current search, as described in the latest paper, Reines returned to radio searches, hunting for radio signatures with that sample. This, she says, should allow her to find massive black holes in star-forming dwarf galaxies, even though she has only found one thus far.
“When new discoveries break our current understanding of the way things work, we find even more questions than we had before,” comments Yves Idzerda, head of the Department of Physics at MSU.
As for Reines, the search continues.
“There are lots of opportunities to make new discoveries because studying black holes in dwarf galaxies is a new field,” she said. “People are definitely captivated by black holes. They’re mysterious and fascinating objects.”
An artist’s conception of two black holes entwined in a gravitational tango. NASA/JPL-Caltech/SwRI/MSSS/Christopher Go.
Do supermassive black holes have friends? The nature of galaxy formation suggests that the answer is yes, and in fact, pairs of supermassive black holes should be common in the universe.
I am an astrophysicist and am interested in a wide range of theoretical problems in astrophysics, from the formation of the very first galaxies to the gravitational interactions of black holes, stars and even planets. Black holes are intriguing systems, and supermassive black holes and the dense stellar environments that surround them represent one of the most extreme places in our universe.
The supermassive black hole that lurks at the center of our galaxy, called Sgr A*, has a mass of about 4 million times that of our Sun. A black hole is a place in space where gravity is so strong that neither particles or light can escape from it. Surrounding Sgr A* is a dense cluster of stars. Precise measurements of the orbits of these stars allowed astronomers to confirm the existence of this supermassive black hole and to measure its mass. For more than 20 years, scientists have been monitoring the orbits of these stars around the supermassive black hole. Based on what we’ve seen, my colleagues and I show that if there is a friend there, it might be a second black hole nearby that is at least 100,000 times the mass of the Sun.
At the center of our galaxy is a supermassive black hole in the region known as Sagittarius A. It has a mass of about 4 million times that of our Sun. ESA–C. Carreau.
Supermassive black holes and their friends
Almost every galaxy, including our Milky Way, has a supermassive black hole at its heart, with masses of millions to billions of times the mass of the Sun. Astronomers are still studying why the heart of galaxies often hosts a supermassive black hole. One popular idea connects to the possibility that supermassive holes have friends.
To understand this idea, we need to go back to when the universe was about 100 million years old, to the era of the very first galaxies. They were much smaller than today’s galaxies, about 10,000 or more times less massive than the Milky Way. Within these early galaxies the very first stars that died created black holes, of about tens to thousand the mass of the Sun. These black holes sank to the center of gravity, the heart of their host galaxy. Since galaxies evolve by merging and colliding with one another, collisions between galaxies will result in supermassive black hole pairs – the key part of this story. The black holes then collide and grow in size as well. A black hole that is more than a million times the mass of our son is considered supermassive.
If indeed the supermassive black hole has a friend revolving around it in close orbit, the center of the galaxy is locked in a complex dance. The partners’ gravitational tugs will also exert its own pull on the nearby stars disturbing their orbits. The two supermassive black holes are orbiting each other, and at the same time, each is exerting its own pull on the stars around it.
The gravitational forces from the black holes pull on these stars and make them change their orbit; in other words, after one revolution around the supermassive black hole pair, a star will not go exactly back to the point at which it began.
Using our understanding of the gravitational interaction between the possible supermassive black hole pair and the surrounding stars, astronomers can predict what will happen to stars. Astrophysicists like my colleagues and me can compare our predictions to observations, and then can determine the possible orbits of stars and figure out whether the supermassive black hole has a companion that is exerting gravitational influence.
Using a well-studied star, called S0-2, which orbits the supermassive black hole that lies at the center of the galaxy every 16 years, we can already rule out the idea that there is a second supermassive black hole with mass above 100,000 times the mass of the Sun and farther than about 200 times the distance between the Sun and the Earth. If there was such a companion, then I and my colleagues would have detected its effects on the orbit of SO-2.
But that doesn’t mean that a smaller companion black hole cannot still hide there. Such an object may not alter the orbit of SO-2 in a way we can easily measure.
The physics of supermassive black holes
Supermassive black holes have gotten a lot of attention lately. In particular, the recent image of such a giant at the center of the galaxy M87 opened a new window to understanding the physics behind black holes.
The first image of a black hole. This is the supermassive black hole at the center of the galaxy M87. Event Horizon Telescope Collaboration, CC BY-SA.
The proximity of the Milky Way’s galactic center – a mere 24,000 light-years away – provides a unique laboratory for addressing issues in the fundamental physics of supermassive black holes. For example, astrophysicists like myself would like to understand their impact on the central regions of galaxies and their role in galaxy formation and evolution. The detection of a pair of supermassive black holes in the galactic center would indicate that the Milky Way merged with another, possibly small, galaxy at some time in the past.
That’s not all that monitoring the surrounding stars can tell us. Measurements of the star S0-2 allowed scientists to carry out a unique test of Einstein’s general theory of relativity. In May 2018, S0-2 zoomed past the supermassive black hole at a distance of only about 130 times the Earth’s distance from the Sun. According to Einstein’s theory, the wavelength of light emitted by the star should stretch as it climbs from the deep gravitational well of the supermassive black hole.
The stretching wavelength that Einstein predicted – which makes the star appear redder – was detected and proves that the theory of general relativity accurately describes thephysics in this extreme gravitational zone. I am eagerly awaiting the second closest approach of S0-2, which will occur in about 16 years, because astrophysicists like myself will be able to test more of Einstein’s predictions about general relativity, including the change of the orientation of the stars’ elongated orbit. But if the supermassive black hole has a partner, this could alter the expected result.
Finally, if there are two massive black holes orbiting each other at the galactic center, as my team suggests is possible, they will emit gravitational waves. Since 2015, the LIGO-Virgo observatories have been detecting gravitational wave radiation from merging stellar-mass black holes and neutron stars. These groundbreaking detections have opened a new way for scientists to sense the universe.
Any waves emitted by our hypothetical black hole pair will be at low frequencies, too low for the LIGO-Virgo detectors to sense. But a planned space-based detector known as LISA may be able to detect these waves which will help astrophysicists figure out whether our galactic center black hole is alone or has a partner.
The idea of stars orbiting the supermassive black holes that researchers believe lurk at the centre of most galaxies has been long established as a matter of fact in science. In ‘active galactic nuclei’ or AGNs, these black holes are surrounded by haloes of gas and dust in a violent churning environment. Such clouds of gas and dust have the potential to birth not only stars but planets as well. Yet, the question of whether planets can also orbit these spacetime events has yet to be established.
Enter Keiichi Wada, a professor at Kagoshima University, and Eiichiro Kokubo, a professor at the National Astronomical Observatory of Japan. These scientists from the distinct fields of active galactic nuclei research and planet formation research respectively have calculated that as a result of gas disc growth, an entirely new class of planets may form around supermassive black holes.
“With the right conditions, planets could be formed even in harsh environments, such as around a black hole,” Wada points out.
In their research published in the Astrophysical Journal, the duo of theoreticians propose that protoplanetary discs that surround young stars may not be the only potential site for planet formation. The researchers instead focused calculations and mathematical models on the denser dust discs found around supermassive black holes in AGNs, thus arriving at a surprising conclusion.
“Our calculations show that tens of thousands of planets with 10 times the mass of the Earth could be formed [at a distance of] around 10 light-years from a black hole,” says Eiichiro Kokubo.
“Around black holes, there might exist planetary systems of astonishing scale.”
One of the hindrances to the formation of planets in such discs of dust has previously been the amount of energy generated in AGNs, Researchers had believed that this energy output would prevent the coagulation of ‘fluffy ice dust’ that can help the growth of dust grains that can lead to planet formation in protoplanetary discs.
But, what Wada and Kokubo discovered was that the huge density of dust discs around supermassive black holes in AGNs —potentially containing as much as a hundred thousand times the mass of the Sun worth of dust, which is a billion times more massive than a typical protoplanetary disc — helps protect the outer layers from bombardment from high-energy radiation such as gamma rays.
This helps form a low-temperature region similar to that found in protoplanetary discs, and thus, in turn, increases the likelihood of fluffy deposits building.
The process would lead to the formation of planets within a period of several hundred million years, according to the pair, and also result in much denser and more populated collections of planets.
Unfortunately, the limits of current methods of identifying exoplanets would make identifying planets around a supermassive black hole challenging to say the least.
“ Doppler spectroscopy, transit photometry, gravitational micro-lensing, or direct imaging are hopeless,” warn the duo in their paper. They go on to suggest that a method called photometry with an x-ray interferometer located in space could be a possible solution — if a way of distinguishing the effect caused by such planets from the natural variability of the AGN can be developed.
For now, researchers will have to look to mathematical models alone to theorise about the potential for planets in orbit around black holes.
Astronomers have discovered a star travelling at an incredible 6 million km/h — ten times faster than the average star — after being ejected by the supermassive black hole at the centre of the Milky Way five million years ago.
Carnegie Mellon University Assistant Professor of Physics Sergey Koposov discovered the star — named S5-HVS1 — as part of the Southern Stellar Stream Spectroscopic Survey (S5).
“The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return,” said Douglas Boubert from the University of Oxford, a co-author of the study.
S5-HVS1 — located in the constellation of Grus — is part of a population of objects known as ‘high-velocity stars’ (HVSs). These stars sparked curiosity amongst astronomers after the first example was discovered in 2005. In the next 14 years, many more examples of HVSs have been uncovered.
But, even amongst these aptly-named stars, S5-HVS1 is exceptional for its high speed. The star’s close passage to Earth at a mere (in astronomical terms) 2.9 x 10⁴ light-years away, also makes it somewhat unique.
Armed with information about the runaway star’s blazing speed coupled with its close proximity has allowed astronomers to track its trajectory back to the centre of the Milky Way and the supermassive black hole — Sagittarius A* (Sgr A*) — which dwells there.
“This is super exciting, as we have long suspected that black holes can eject stars with very high velocities. However, we never had an unambiguous association of such a fast star with the galactic centre,” says Koposov, the lead author of this work and member of Carnegie Mellon’s McWilliams Center for Cosmology. “We think the black hole ejected the star with a speed of thousands of kilometres per second about five million years ago.
“This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.”
A bad break-up?
So how on Earth did S5-HVS1 come to be travelling at such an extraordinary speed?
Astronomers believe that the star was once part of a binary system with a companion star. It was ejected from this partnership after both stars’ orbits strayed too close to Sgr A*. Whilst its partner was captured by the incredible gravitational attraction of the supermassive black hole, the gravitational struggle tore S5-HVS1 free and launched it on its rapid journey.
This process is known as the ‘Hills mechanism’ and was first suggested by astronomer Jack Hills thirty years ago and has long been considered as a likely mechanism for the origins of high-velocity stars.
“This is the first clear demonstration of the Hills Mechanism in action,” points out Ting Li from Carnegie Observatories and Princeton University, and leader of the S5 Collaboration. “Seeing this star is really amazing as we know it must have formed in the galactic centre, a place very different from our local environment.
“It is a visitor from a strange land.”
An exceptional observation
The astronomers made the discovery of S5-HVS1 was made with 3.9-metre Anglo-Australian Telescope (AAT) near Coonabarabran, NSW, Australia. The team was only able to assess the true speed of the star and details of its incredible journey when these observations were coupled with further data from the European Space Agency’s Gaia satellite.
“The observations would not be possible without the unique capabilities of the 2dF instrument on the AAT,” adds Daniel Zucker, an astronomer at Macquarie University in Sydney, Australia, and a member of the S5 executive committee. “It’s been conducting cutting-edge research for over two decades and still is the best facility in the world for our project.”
The team’s results are published in the journal Monthly Notices of the Royal Astronomical Society.
“I am so excited this fast-moving star was discovered by S5,” says Kyler Kuehn, at Lowell Observatory and a member of the S5 executive committee. “While the main science goal of S5 is to probe the stellar streams — disrupting dwarf galaxies and globular clusters — we dedicated spare resources of the instrument to searching for interesting targets in the Milky Way, and voila, we found something amazing for ‘free.’
“With our future observations, hopefully, we will find even more!”
Like virtually all other galaxies, the Milky Way houses a supermassive black hole at its center with a mass millions of times greater than the sun. It constantly gobbles up enormous amounts of gas and dust from its surroundings but scientists have noticed that even for its gargantuan appetite, this black hole is now unusually hungry.
“We have never seen anything like this in the 24 years we have studied the supermassive black hole,” said Andrea Ghez, UCLA professor of physics and astronomy and a co-senior author of the research. “It’s usually a pretty quiet, wimpy black hole on a diet. We don’t know what is driving this big feast.”
The astronomers employed more than 13,000 observations of the black hole, called Sagittarius A*, performed by the W.M. Keck Observatory in Hawaii and the European Southern Observatory’s Very Large Telescope in Chile since 2003.
Although black holes cannot be imaged directly since their massive gravity allows nothing to escape their grasp, not even light, astronomers can see brightness at the edge of the black hole’s point of no return — the event horizon.
The brightness is generated by radiation from gas and dust that are accelerated to huge speeds as they circle the event horizon. Moving at close to the speed of light, the matter generates powerful jets of plasma containing electrons and positrons that ricochet off the event horizon and get hurled outward along the black hole’s axis of rotation. It is these enormous jets of energetic subatomic particles that emit light, which our telescopes can see. Last year, researchers were able to produce the first image of a black hole’s event horizon.
Observations of Sagittarius A* performed on May 13 showed that its brightness had enhanced considerably.
“The first image I saw that night, the black hole was so bright I initially mistook it for the star S0-2, because I had never seen Sagittarius A* that bright,” said UCLA research scientist Tuan Do, the study’s lead author. “But it quickly became clear the source had to be the black hole, which was really exciting.”
Astronomers aren’t sure what’s gone into Sagittarius A* but it’s possible that the extreme brightness swings may have been triggered after a nearby star called S0-2 made a close approach to the black hole during 2018. In the process, it should have discharged a large quantity of gas.
Another hypothesis is that G2, a binary star system was stripped off its outer layer during a close approach to the black hole in 2014. This may explain the black hole’s sudden variations in brightness just outside of it.
Alternatively, very large asteroids that ventured too close to the black hole may have contributed to the jump in brightness, the astronomers noted in a recent paper published in The Astrophysical Journal.
Whatever may be the case, the anomaly is no threat to life on Earth. The black hole is located about 26,000 light-years away from Earth and its radiation would have to be 10 billion times brighter to affect us in a significant way.
What’s perhaps more fascinating than this anomaly, however, is the way the astronomers were able to track the black hole’s feeding patterns all of these years.
In 2004, Ghez and colleagues helped pioneer a crucial piece of technology, called adaptive optics, which corrects the distorting effects of Earth’s atmosphere in real-time. This allowed them to observe more than 3,000 stars in the vicinity of Sagittarius A*.
However, astronomers have been observing Sagittarius A* since long before adaptive optics were invented. In order to make good use of observations made prior to 2004, Ghez developed a new technique called speckle holography that can use faint information from 24 years of data recorded on the black hole and fill in the blanks. The technique was recently described in The Astrophysical Journal Lettersand allowed the researchers to determine that the black hole’s brightness is at an all-time high since we’ve been observing it.
Remarkably, the technique also allowed the astronomers to test Einstein’s theory of general relativity near the black hole. The researchers observed the black hole’s effects on S0-2 at it completed an orbit; effects which mirrored Einstein’s predictions.
“It was like doing LASIK surgery on our early images,” Ghez said. “We collected the data to answer one question and serendipitously unveiled other exciting scientific discoveries that we didn’t anticipate.”
Researchers have turned to a massive supercomputer — dubbed the ‘UniverseMachine’ — to model the formation of stars and galaxies. In the process, they created a staggering 8 million ‘virtual universes’ with almost 10¹⁴ galaxies.
To say that the origins and evolution of galaxies and the stars they host have been an enigma that scientists have sought to explore for decades is the ultimate understatement.
In fact, desire to understand how the stars form and why they cluster the way they do, predates science, religion and possibly civilisation itself. As long as humans could think and reason — way before we knew what either a ‘star’ or a ‘galaxy’ was— we looked to the heavens with a desire to have knowledge of its nature.
We now know more than we ever have, but the heavens and their creation still hold mysteries for us. Observing real galaxies can only provide researchers with a ‘snapshot’ of how they appear at one moment. Time is simply too vast and we exist for far too brief a spell to observe galaxies as they evolve.
Now a team of researchers led by the University of Arizona have turned to supercomputer simulations to bring us closer to an answer for these most ancient of questions.
Astronomers have used such computer simulations for many years to develop and test models of galactic creation and evolution — but it only works for one galaxy at a time — thus failing to provide a more ‘universal’ picture.
To overcome this hurdle, Peter Behroozi, an assistant professor at the UA Steward Observatory, and his team generated millions of different universes on a supercomputer. Each universe was programmed to develop with a separate set of physical theories and parameters.
As such the team developed their own supercomputer — the UniverseMachine, as the researchers call it —to create a virtual ‘multiverse’ of over 8-million universes and at least 9.6 x 10¹³ galaxies.
The results could solve a longstanding quirk of galaxy-formation — why galaxies cease forming new stars when the raw material — hydrogen — is not yet exhausted.
The study seems to show that supermassive black holes, dark matter and supernovas are far less efficient at stemming star-formation than currently theorised.
The team’s findings — published in the journal Monthly Notices of the Royal Astronomical Society — challenges many of the current ideas science holds about galaxy formation. In particular, the results urge a rethink of how galaxies form, how they birth stars and the role of dark matter — the mysterious substance that makes up 80% of the universe’s matter content.
Behroozi, the study’s lead author. says: “On the computer, we can create many different universes and compare them to the actual one, and that lets us infer which rules lead to the one we see.”
What makes the study notable is it is the first time each universe simulated has contained 12 million galaxies, spanning a time period of 400 million years after the ‘big bang’ to the present day. As such, the researchers have succeeded in the creation of self-consistent universes which closely resemble our own.
Putting the multiverse to the test — how the universe is supposed to work
To compare each universe to the actual universe, each was put through a series of tests that evaluated the appearance of the simulated galaxies they host in comparison to those in the real universe.
Common theories of how galaxies form stars involve a complex interplay between cold gas collapsing under the effect of gravity into dense pockets giving rise to stars. As this occurs, other processes are acting to counteract star formation.
For example, we believe that most galaxies harbour supermassive black holes in their centres. Matter forming accretion discs around these black holes and eventually being ‘fed’ into them, radiate tremendous energies. As such, these systems act almost as a ‘cosmic blowtorch’ heating gas and preventing it from cooling down enough to collapse into stellar nurseries.
Supernova explosions — the massive eruption of dying stars — also contribute to this process. In addition to this, dark matter provides most of the gravitational force acting on the visible matter in a galaxy — thus, pulling in cold gas from the galaxy’s surroundings and heating it up in the process.
Behroozi elaborates: “As we go back earlier and earlier in the universe, we would expect the dark matter to be denser, and therefore the gas to be getting hotter and hotter.
“This is bad for star formation, so we had thought that many galaxies in the early universe should have stopped forming stars a long time ago.”
But what the team found was the opposite.
Behroozi says: “Galaxies of a given size were more likely to form stars at a higher rate, contrary to the expectation.”
Bending the rules with bizarro universes
In order to match observations of actual galaxies, the team had to create virtual universes in which the opposite was the case — universes in which galaxies continued to birth stars for much longer.
Had the researchers created universes based on current theories of galaxy formation — universes in which the galaxies stopped forming stars early on — those galaxies appeared much redder than the galaxies we see in the sky.
Galaxies appear red for major two reasons. If the galaxy formed earlier in the history of the universe cosmic expansion — the Hubble flow — means that it will be moving away from us more rapidly, causing significant elongation in the wavelength of the light it emits shifting it to the red end of the electromagnetic spectrum. A process referred to as redshift.
In addition to this, another reason an older galaxy may appear red is intrinsic to that galaxy and not an outside effect like redshift. If a galaxy has stopped forming stars, it will contain fewer blue stars, which typically die out sooner, and therefore be left with older — redder — stars.
Behroozi point out that isn’t what the team saw in their simulations, however. He says: “If galaxies behaved as we thought and stopped forming stars earlier, our actual universe would be coloured all wrong.
“In other words, we are forced to conclude that galaxies formed stars more efficiently in the early times than we thought. And what this tells us is that the energy created by supermassive black holes and exploding stars is less efficient at stifling star formation than our theories predicted.”
Computing the multiverse is as difficult as it sounds
Creating mock universes of unprecedented complexity required an entirely new approach that was not limited by computing power and memory, and provided enough resolution to span the scales from the “small” — individual objects such as supernovae — to a sizeable chunk of the observable universe.
Behroozi explains the computing challenge the team had to overcome: “Simulating a single galaxy requires 10 to the 48th computing operations. All computers on Earth combined could not do this in a hundred years. So to just simulate a single galaxy, let alone 12 million, we had to do this differently.”
In addition to utilizing computing resources at NASA Ames Research Center and the Leibniz-Rechenzentrum in Garching, Germany, the team used the Ocelote supercomputer at the UA High-Performance Computing cluster.
Two-thousand processors crunched the data simultaneously over three weeks. Over the course of the research project, Behroozi and his colleagues generated more than 8 million universes.
He explains: “We took the past 20 years of astronomical observations and compared them to the millions of mock universes we generated.
“We pieced together thousands of pieces of information to see which ones matched. Did the universe we created look right? If not, we’d go back and make modifications, and check again.”
Behroozi and his colleagues now plan to expand the Universe Machine to include the morphology of individual galaxies and how their shapes evolve over time.
As such they stand to deepen our understanding of how the galaxies, stars and eventually, life came to be.
Measurements of a star passing close to the supermassive black hole at the centre of the Milky Way confirms the predictions of Einstein’s theory of general relativity in a high gravity environment.
An artist visualization of the star S0–2 as it passes by the supermassive black hole at the Galactic centre. As the star gets closer to the supermassive black hole, light it emits experiences a gravitational redshift that is predicted by Einstein’s General Relativity. By observing this redshift, we can test Einstein’s theory of gravity (Nicole R. Fuller, National Science Foundation)
A detailed study of a star orbiting the supermassive black hole at the centre of our Galaxy, reveals that Einstein’s theory of general relativity is accurate in its description of the behaviour of light struggling to escape the gravity around this massive space-time event.
The analysis — conducted by Tuan Do, Andrea Ghez and colleagues — involved detecting the gravitational redshift in the light emitted by a star closely orbiting the supermassive black hole known as Sagittarius A*. The redshift was measured as the star reached the closest point in its orbit — which has a duration of 16 years — to the black hole.
The team found that the star experienced gravitational redshift — which occurs when light is stretched to longer wavelengths and towards the red ‘end’ of the electromagnetic spectrum by the effect of gravity — as it gets closer to the black hole, conforming to Einstein’s theory of general relativity and its predictions regarding gravity.
At the same time, the results defy predictions made by the Newtonian theory, which has no explanation for gravitational redshift.
Ghez says: “(The findings are) a transformational change in our understanding about not only the existence of supermassive black holes but the physics and astrophysics of black holes.”
The major difference between general relativity and the Newtonian calculation of gravity is, that whereas Newton envisioned gravity as a force acting between physical objects, Einstein’s theory saw gravity as a geometric phenomenon.
The presence of mass ‘curves’ space it occupies. Physical objects, including light, must then follow this curvature. As John Wheeler infamously put it: “matter tells space how to curve, space tells matter how to move.”
Testing relativity in regions of high gravity
The new research resembles an analysis conducted last year by the GRAVITY collaboration, except in this new expanded analysis, the team report novel spectra data.
Although general relativity has been thoroughly tested in relatively weak gravitational fields — such as those on Earth and in the Solar System—before last year, it had not been tested around a black hole as big as the one at the centre of the Milky Way.
Observations of the stars rapidly orbiting Sagittarius A *provide a method for general relativity to be evaluated in an extreme gravitational environment.
Do explains why these kind of tests are important:
“We need to test GR in extreme environments because that’s where we think the theory might break down.”
“If we can see which predictions from general relativity have deviations, that gives us clues as to how to build a better model of gravity.”
To obtain their results, the team analyzed new observations of the star S0–2 as it made its closest approach to the enormous black hole in 2018. They then combined this data with measurements Ghez and her team have made over the last 24 years.
The team has many avenues of investigations available to them from here, Tuan tells me.
He continues: “Two of them I’m excited about are testing space-time around the black hole by looking at the orbit of the star S0–2.”
“GR predicts that the orbit should precess, or rotate, meaning that it won’t come back where it started.”
The team should also be able to start using more stars other than S0–2 for these tests as the time baseline of observations increase and technology improves
Do concludes: “ These measurements open a new era of GR tests at the Galactic centre so it’s very exciting.”
This research appears in the 26 July 2019 issue of Science.
The black hole at the center of our galaxy, Sagittarius A*. Credit: J.Davelaar 2018.
Gravity is so strong in the vicinity of a black hole that nothing can escape its clutches, not even light. This makes that black holes very difficult to study (we aren’t able to directly image them) — they’re also incredibly frightening. Luckily, you don’t have to get spaghettified in order to see what a black hole’s environment looks like. An international team of researchers has created an immersive virtual reality simulation based on recent astrophysical models of Sagittarius A* — the supermassive black hole at the center of the Milky Way — and it all looks as amazing as it sounds.
“Our virtual reality simulation creates one of the most realistic views of the direct surroundings of the black hole and will help us to learn more about how black holes behave. Traveling to a black hole in our lifetime is impossible, so immersive visualizations like this can help us understand more about these systems from where we are,” Jordy Davelaar, a postgrad at Radboud University, The Netherlands, and co-author of the new study, said in a statement.
The researchers employed the most advanced physical models to create a series of images that were stitched together, forming a 360-degree virtual reality simulation of Sagittarius A*. Using any widely available VR console, people can now dive deep into the chaos that surrounds the black hole.
Sagittarius A* is located around 26,000 light years from Earth, and scientists estimate it measures 27 million miles (44 million km) across. The black hole itself — or rather its event horizon — cannot be imaged by telescopes due to large clouds of gas and dust blocking the view.
About 1% of all matter that orbits the black hole is sucked in. The rest forms a swirling disk many light-years across that constitutes a reservoir of gas and dust that black holes feed on — and its motion is completely mesmerizing!
The realistic simulation will help researchers form a better understanding of the dynamics of black hole systems. It also serves an educational purpose by providing a compelling visual explanation of how black holes behave.
“The visualisations that we produced have a great potential for outreach. We used them to introduce children to the phenomenon of black holes, and they really learned something from it. This suggests that immersive virtual reality visualizations are a great tool to show our work to a broader audience, even when it involves very complicated systems like black holes,” Davelaar said.
For the first time, astronomers have observed pairs of galaxies in the final stages of merging into a single, larger galactic body. The findings suggest that such events are more common than astronomers used to think.
The final stage of a union between two galactic nuclei in the messy core of the merging galaxy NGC 6240. Credit: NASA, ESA, W. M. Keck Observatory, Pan-STARRS and M. Koss.
Astronomers suspect that supermassive black holes lurk at the heart of every sizable galaxy, holding the galactic fiber together. For instance, at the galactic core of the Milky Way, there should be a supermassive black hole called Sagittarius A*, a staggering 4.5 million solar masses in size.
Black holes likely reach this sort of dizzying size through the merger of galaxies. However, evidence has been conflicting due to a lack of direct imaging of the process, which is obscured by swirling clouds of gas and dust. What’s more, simulations show that the more these galaxies progress in their merger, the greater the concealment effect.
To peer through all the matter that obscures supermassive black holes, scientists combed through a huge catalog of 10 years’ worth of X-ray measurements taken by the Burst Alert Telescope (BAT) aboard NASA’s Neil Gehrels Swift Observatory.
“The advantage to using Swift’s BAT is that it observes high-energy, ‘hard’ X-rays,” said study co-author Richard Mushotzky, a professor of astronomy at the University of Maryland. “These X-rays penetrate through the thick clouds of dust and gas that surround active galaxies, allowing the BAT to see things that are literally invisible in other wavelengths.”
Next, the research team analyzed another catalog of galaxies from NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii whose X-ray signatures matched the Swift readings.
Various colliding galaxies along with closeup views of their coalescing nuclei in the bright cores. The images were taken in near-infrared light by the Keck Observatory in Hawaii. Credit: Keck images: W. M. Keck Observatory and M. Koss.
The breakthrough lied with the Keck Observatory’s adaptive optics technology whose deformable mirrors controlled by a computer enable a phenomenal increase in resolution. Using this tech, researchers were able to produce extremely sharp, near-infrared images of X-ray-producing black holes not found in the Hubble archive.
“People had conducted studies to look for these close interacting black holes before, but what really enabled this particular study were the X-rays that can break through the cocoon of dust,” explained Koss. “We also looked a bit farther in the universe so that we could survey a larger volume of space, giving us a greater chance of finding more luminous, rapidly-growing black holes.”
In total, 96 galaxies were observed with the Keck telescope and 385 galaxies from the Hubble archive. According to the results, 17 percent of these galaxies host a pair of black holes at their center, which are locked in the late stages of a galactic merger. This was surprising to learn since previously simulations suggested that black hole pairs spend very little time in this phase.
“Seeing the pairs of merging galaxy nuclei associated with these huge black holes so close together was pretty amazing,” said Michael Koss, co-author of the new study published in the journal Nature. “In our study, we see two galaxy nuclei right when the images were taken. You can’t argue with it; it’s a very ‘clean’ result, which doesn’t rely on interpretation.”
The findings suggest that galactic mergers may indeed be a key process by which black holes grow to stupendous masses. And the stages that the astronomers have mapped out in this study likely foretell the fate of our very own galaxy. In about 6 billion years, scientists estimate that the Milky Way will merge with the Andromeda galaxy into one big galaxy.
There is still much to learn about black hole mergers, though. Right now, hardware is a huge limitation, but, once the James Webb Space Telescope is deployed in 2021, scientists will be able to measure masses, growth rates, and other physical parameters of black hole pairs.
Illustration of hot clumps of gas that orbit the black hole at the center of the Milky Way. Credit: ESO/Gravity Consortium/L. Calçada.
Scientists have known for a long time that at the very heart of the Milky Way lies a supermassive black hole, about four million times more massive than the Sun. As its name suggests, we can’t image a black hole directly, but cutting-edge telescopes can tease out the infrared light emitted by interstellar gas as it swirls into the black hole. Now, an international team of researchers led by the Max Planck Institute for Extraterrestrial Physics reported evidence of knots of gas that appear to orbit the galactic center. This remarkable observation is the closest look yet at our galactic supermassive black hole and, at the same time, offers new opportunities to test the laws of physics.
The point of no return
To image things in the vicinity of Sagittarius A*, the Milky Way’s supermassive black hole, researchers looked to the GRAVITY project. Using a special technique called interferometry, four eight-meter-wide telescopes at the European Southern Observatory’s Very Large Telescope in Chile were combined to produce images that only a hypothetical telescope as large as entire countries could produce. By the same technique, in the future, a ‘planet-sized’ instrument called the Event Horizon Telescope hopes to produce an actual image of a black hole.
The new observations measured the brightness and position of infrared flares in the vicinity of Sagittarius A*. These flares actually trace a tiny circle in the night’s sky, the researchers found, moving clockwise.
Yepun telescope, part of the European Southern Observatory’s (ESO’s) Very Large Telescope. Credit: ESO.
These kinds of outbursts had been detected before. However, this was the first time that astronomers precisely measured the flares’ positions and motions before they dissipated. Each flare moved at about 30% light speed in a 45-minute orbit around what we can only suppose is a black hole.
Earlier this year, the same team measured the relativistic distortion of light from a star, S2, during its closest approach to Sagittarius A*.
These hot spots might be produced by shock waves in magnetic fields, much as solar flares erupt from the sun. Due to the immense gravitational forces present in the vicinity of the black hole, space-time itself is twisted into something resembling a lens, which causes these hot spots, circling at 30% the speed of light, to flash beacons of light across the cosmos. And by further studying these flares, researchers hope to tease out the black hole’s spin or rotation.
All of this, of course, assuming Einstein’s general theory of relativity is correct, which implies that the orbits of objects around a black hole are determined solely by the black hole’s mass and spin. If not, then the theory might need some refinement to accommodate for any observed inconsistencies.
Nearly 150 million light-years away from Earth, two distant galaxies are colliding in a tragic dance that will ignite billions of suns like fireworks. At the heart of one of these galaxies, however, there’s another violent event playing out. For the first time, astronomers have directly imaged a supermassive black hole ejecting a fast-moving jet of particles as it shreds a passing star.
Artist impression of Tidal Disruption Event (TDE) in Arp 299. The background image is a Hubble Space Telescope image of Arp 299, a pair of colliding galaxies. Credit: Sophia Dagnello, NRAO/AUI/NSF; NASA, STScI.
Supermassive black holes aren’t your ordinary stellar-variety black holes whose mass is just a couple of times that of our sun. Instead, these objects can have millions or, in some extreme cases, billions of solar masses. Virtually every galaxy has a supermassive black hole lurking at its center, which directly influences its development. Our own galaxy, the Milky Way, is no exception.
A supermassive black hole’s huge gravitational pull will actively draw material from its surroundings, but not all of it will be gobbled in at once. Instead, the black hole will form a rotating disc around it and launch superfast jets of particles from the poles of the disk at nearly the speed of light.
In 2005, astronomers working with the William Herschel Telescope in the Canary Islands identified a bright burst of infrared emission coming from the nucleus of one of the colliding galaxies in Arp 299. A bit later, the National Science Foundation’s Very Long Baseline Array (VLBA) revealed a new, distinct source of radio emission from the same location.
“As time passed, the new object stayed bright at infrared and radio wavelengths, but not in visible light and X-rays,” said Seppo Mattila, of the University of Turku in Finland in a statement. “The most likely explanation is that thick interstellar gas and dust near the galaxy’s center absorbed the X-rays and visible light, then re-radiated it as infrared,” he added.
Continued observations with the VLBA and other radio telescopes confirmed the presence of a source of radio emissions expanding in one direction, typical of a jet, just like scientists expected.
The Very Long Baseline Array (VLBA) is a massive interferometer consisting of 10 identical antennas on transcontinental baselines spanning up to 8,000 km, from Mauna Kea, Hawaii to St. Croix, Virgin Islands. These multiple radio antennas separated by thousands of kilometers allow the VLBA to gain an incredible resolving power — the ability to see fine detail — which is required to observe the features of an expanding object from millions of light-years away. The VLBA observes at wavelengths of 28 cm to 3 mm (1.2 GHz to 96 GHz) in eight discrete bands plus two narrow sub-gigahertz bands, including the primary spectral lines that produce high-brightness maser emission.
According to the researchers, the jet is emitted by a supermassive black hole, located at the heart of one of the colliding galaxies pairs called Arp 299. The black hole is about 20 million times more massive than the Sun and is currently shredding a star over two times as massive as the Sun, that was unfortunate enough to drift too close to the gargantuan monster. The superfast jet of charged particles emitted by the black hole packs a staggering 125 billion times the amount of energy the sun releases per year.
Only a small number of such stellar deaths called tidal disruption events, or TDEs, have been detected. The bursts propagate all over the electromagnetic spectrum, from radio, visible, and UV all the way to X-ray and gamma-ray intervals.
“Never before have we been able to directly observe the formation and evolution of a jet from one of these events,” said Miguel Perez-Torres, of the Astrophysical Institute of Andalusia in Granada, Spain.
Because the dust around the black hole absorbed any visible light, this particular TDE might be indicative of a hidden population of similar events — the tip of the iceberg, if you will. Mattila and Perez-Torres hope to discover many more such events and learn from them by directed infrared and radio telescopes to candidate sources.
TDEs are important to astronomy since they provide unique insight into the formation and evolution of jets in the vicinity of massive objects. Such events are likely common in the distant universe and studying them will advance our understanding of galaxies that developed billions of years ago.