Astronomers have never observed a black hole directly. We know they exist. We can see the powerful jets produced by active supermassive black holes. We can see stars orbiting the black hole in the center of the Milky Way. We can also observe the gravitational waves produced when two black holes merge. What we haven’t observed is the region just outside a black hole’s event horizon. But the Event Horizon Telescope is now trying to do just that. 

Participating observatories of the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA). Credit: ESO/O. Furtak

The Event Horizon Telescope (EHT) isn’t a single telescope, but rather a collection of radio telescopes working together from all over the world. Each will observe the center of our galaxy at a wavelength of about 1.3 millimeters. This wavelength was chosen because it is a wavelength where the gas and dust surrounding the center of our galaxy is relatively transparent.

But the greatest challenge to observing a black hole is its size. The center of our galaxy is 26,000 light years way, and while the supermassive black hole it contains has a mass of about 4 million Suns, it is only 20 times wider than the Sun. To see the region around such a small object requires an extremely high resolution telescope. More than a single radio telescope can provide. But by combining observations from telescopes all over the world, we can create a “virtual telescope” the size of Earth itself. In this way we can get a resolution down to about 50 microarcseconds.

If the project is successful, it will give us a direct view of the region around a black hole, and this will allow us to test whether our understanding of black holes is correct. For example, the light coming from just outside the black hole should be caused by synchrotron radiation, so it will be highly polarized. According to general relativity, the warped space around the black hole will twist the polarized light in ways we can measure. By observing the gravitational effect on light, we can test whether general relativity is correct.

What’s perhaps most amazing about this project is that it is perhaps the largest collaborative astronomy project we’ve ever undertaken. Countries from from all over the world have come together, all to try to observe one of the smallest objects in our galaxy.

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Most galaxies like our Milky Way have a supermassive black hole in their core. They can have more mass than millions or billions of Suns. But there are a few spiral galaxies, such as the Triangulum galaxy, that has no supermassive black hole. Given that supermassive black holes form within galaxies, how could a galaxy possibly get rid of one? The answer could be gravitational waves. 

Every now and then two galaxies can collide and merge. For example, the Andromeda galaxy and Milky Way are moving toward each other, and will collide in about 4 billion years. When two galaxies collide, their supermassive black holes can be caught in a spiral toward each other. As they orbit each other, their large masses produce strong gravitational waves, which eventually causes them to merge.

A diagram showing how two merging black holes can be ejected from a galaxy. Credit: NASA, ESA/Hubble, and A. Feild/STScI

We’ve observed the gravitational waves of merging black holes, but the gravitational waves of supermassive black holes would be much stronger. If the two merging black holes are roughly the same size, then the gravitational waves would be generated evenly in a range of directions. But if the black holes have very different sizes, then gravitational waves would be produced more strongly in one direction than others. This means more gravitational energy is being emitted in a particular direction, and as a result the black holes would be kicked in the opposite direction. This could be strong enough to kick a supermassive black hole out of the merged galaxy.

At least that’s been one idea, and new observational evidence seems to support it. A team using the Hubble space telescope have found a supermassive black hole moving away from a galaxy. You can see this as the bright object in the center of image at the top of the page. The black hole is bright because it’s actively consuming gas and dust. Behind the black hole is the fainter and diffuse galaxy 3C186. Given the structure of 3C186, it is most likely the result of two merged galaxies. This is exactly what we’d expect to see when merging black holes are kicked out of a galaxy.

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Inside the galaxy known as NGC 1600 there is a black hole 17 billion times more massive than our Sun. It’s been heralded as the largest black hole ever discovered. While that can be debated, it is certainly among the very largest. It’s also unusual because it doesn’t lie in a dense region of galaxies, but a fairly deserted region. Just how such a large black hole could have formed there isn’t entirely clear.

Since black holes grow in mass by capturing nearby matter, one would expect large black holes to be located in regions that are fairly dense with matter. This is similar to expecting skyscrapers to be located in dense cities, as Jens Thomas (lead author of a new paper on the discovery) puts it. But NGC 1600 is a fairly diffuse elliptical galaxy, and it’s pretty isolated from any nearby galaxies. Finding such a massive black hole in NGC 1600 is like finding a skyscraper in the middle of remote farmland.

Since this black hole is in a diffuse region of the cosmos, it likely won’t grow much beyond its current mass. It’s also likely that the black hole gained its mass much earlier in the Universe, which has implications for just how supermassive black holes formed in the early Universe.

Paper: Jens Thomas, et al. A 17-billion-solar-mass black hole in a group galaxy with a diffuse core. Nature (2016) doi:10.1038/nature17197

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[av_video src=’https://youtu.be/AcAMw29zXx0′ format=’16-9′ width=’16’ height=’9′]

Our Milky Way galaxy is more than 100,000 light years across, and contains more than 200 billion stars. An ultracompact dwarf galaxy has only about 100 million stars, but they are packed into a region only 200 light years across. In such a galaxy you might see a million stars with the naked eye.

The first ultracompact dwarf galaxy (UCD) was discovered last year. Since then a handful of others have been found. From these new discoveries we now have an idea of how they could have formed. UCDs are not only extremely dense with stars, but at least one has a supermassive black hole in its core. This black hole is of a size typically found in larger galaxies like our Milky Way. So it’s thought that UCDs were once larger galaxies that have been stripped of their outer stars due to a gravitational collision with another galaxy, as simulated in the video. There are two clues that support this idea. The first is that these ultracompact dwarfs are each found near a large galaxy that could have stripped away outer stars. The second is that UCDs contain higher than expected amounts of iron in their spectra. Iron is more readily produced in larger galaxies, so this points to UCDs once being a large galaxy. If this idea is true, then the other UCDs should also have supermassive black holes, which will be a focus of future study.

These compact galaxies are both bright and dense, so they are actually pretty easy to observe. Since we didn’t expect such galaxies to exist, they weren’t really noticed until we knew what to look for. Which just goes to show that sometimes a new discovery can be hiding in plain sight.

Paper: Michael A. Sandoval, et al. Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey. Astrophysical Journal Letters, 808, L32. (2015)

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The movie Interstellar features a fast rotating, supermassive black hole known as Gargantua. The Hollywood imagery of Gargantua, with its immense gravity warping light around it in a realistic way, has been praised for its integration of real science into cinematic storytelling. But despite it’s realism, Hollywood enhancement still played a role. Now a new paper in Classical and Quantum Gravity peels back the illusion and shows the Hollywood star without makeup or Photoshop.

Using ray bundles to determine the view.

The paper itself is freely available, so it’s worth checking out. Much of it details the challenges of rendering the black hole at IMAX resolutions. Since the film required Gargantua to be rotating at nearly the maximum theoretical rate, the bending of light is effected by the rotation through what is known as frame dragging. The black hole is also surrounded by a disk of gas and dust, which orbits the black hole. The hot material gives a source of light so that the black hole is visible, but its motion means that the authors had to calculate both how the material moved and how its light was distorted dynamically. Because of the complexity, the team couldn’t use “ray tracing,” where the path of a single beam of light is calculated. Instead they used ray bundles to approximate the effect. Throw the location and motion of the “camera” into the mix and even the approximation is extremely complex.

The paper also discusses the tension between the scientific desire for realism and the Hollywood desire for visual appeal. Because the black hole is rotating so quickly, and the gravity near the black hole is so strong, they greatly affect the visible light around it. The rotational motion of the material creates a Doppler effect, causing the material rotating toward the camera to be shifted toward the blue end of the spectrum, while the material rotating away from the camera appears deep red. This is combined with the fact that light from the approaching side appears much brighter than the receding side. As a result, the fairly accurate rendering shown above lacks visual appeal. So Interstellar‘s director Christopher Nolan opted for a version where the Doppler and brightness effects were minimized. It was also decided that blurring and lens flare effects were added to make it more in line with the overall movie.

Easy, breezy, beautiful. Credit: Interstellar.

It’s hard to be too critical of the cinematic version. Just as the actors are enhanced through makeup and lighting to make them more attractive to audiences, the black hole imagery was enhanced for visual appeal. But the enhancement was done on top of a real science. That’s a big change from the usual approach of simply making things up regardless of what science tells us. Hopefully Interstellar will demonstrate that science fiction movies can aspire for scientific accuracy while still creating interesting tales.

Paper: Oliver James et al. Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar. Class. Quantum Grav. 32 065001 (2015)

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In the center of our galaxy there is a supermassive black hole known as Sgr A*. Through observations of stars orbiting the black hole, we know it has a mass of about 4 million Suns. Normally this black hole is pretty quiet, but in 2013 there was an unexpected x-ray burst.

What’s interesting about this burst is the shortness of its duration. In just a couple of hours Sgr A* brightened by a factor of 400 before fading to normal levels. Given the duration it is possible that an asteroid-sized object was captured by the black hole, creating a burst of energy before crossing its event horizon. Simulations of such captures indicate that the body would be ripped apart by tidal forces and greatly heated before final capture, which would produce an x-ray burst lasting an hour or so. Another idea is that it could be due to magnetic field lines in the black hole’s accretion disk snapping into realignment after being twisted around the black hole. We see similar effects with the magnetic field lines of the Sun and other stars.

Time lapse of the x-ray burst.

It’s difficult to be sure, because Sgr A* is hidden behind a wall of gas and dust in the center of our galaxy known as the zone of avoidance. This means visible light from that region is blocked, and we can only observe Sgr A* with radio waves and x-rays or gamma rays. It isn’t possible to detect small bodies such as asteroids or planets orbiting the black hole.

If other small bodies are heading for the black hole, we won’t know until after it’s happened.

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In the center of our Milky Way galaxy is a supermassive black hole. We can’t see this black hole directly because there is too much dust in the direction of galactic center, but radio waves can penetrate that dust, so we can observe the radio signals of hot stars and gas near galactic center. We’ve been observing these signals over several years, and we’ve noticed how the stars near galactic center orbit the region very quickly. From their orbital motion and a simple use of Kepler’s laws we can get a pretty good idea of the mass of the black hole. It turns out to be about 4 million solar masses. While this is a huge black hole, most of the stars orbiting it aren’t too terribly close. So for the time being they aren’t at risk of being ripped apart by the intense forces near the black hole. But there was one object recently that did make a very close approach.

This particular object is known as G2. First discovered in 2012, its projected orbit would take it within 36 light hours of the black hole at closest approach, which is only about 1,500 times the diameter of the black hole itself. At this distance the tidal forces of the black hole would likely rip G2 apart. This caused a great deal of excitement, since we’ve never seen a black hole destroy an object. Initial studies of G2 seemed to imply that it was a gas cloud of about three Earth masses. It was difficult to be sure because we can only really see it by radio wave emissions. Such a cloud would easily be ripped apart by the black hole, so studies focused on observing the process in action. Even early on we could see that the cloud was being disrupted as it approached the black hole.

But this Summer when the object made its closest approach, it didn’t rip apart. The latest observations, as published this month in the Astrophysical Journal, have shown that G2 remains intact. This is not only surprising, it means that G2 is nothing like what we thought. A diffuse gas cloud would have been ripped apart, so G2 can’t be diffuse. From the latest observations it seems to be a bright star surrounded by a thick region of dust.

So how could we have been so wrong about G2? Part of it could be due to the thick layers of dust around the star itself, but at radio frequencies the dust shouldn’t affect the brightness too much. So the authors propose another solution. Perhaps G2 has been a close binary star, and these stars have recently merged into a larger and brighter star. The close orbit of G2 with the black hole could even have accelerated the merger process, through something known as the Kozai mechanism. If G2 is a recent merger, that would explain both the level of dust around the star as well as a dimmer than expected appearance.

So rather than ripping G2 apart, the supermassive black hole in our galaxy may have worked to hold G2 together.

Paper: G. Witzel, et al. Detection of Galactic Center source G2 at 3.8 μm during periapse passage. ApJ 796 L8 (2014)

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Most galaxies have a supermassive black hole in their centers, but some don’t. The Triangulum galaxy (also known as M33) doesn’t have one, despite being a pretty standard looking spiral galaxy. The general thought is that such galaxies did have a supermassive black hole at one time, but it was ejected by some mechanism. One mechanism is through collisions with other galaxies, but there are other possible mechanisms as well. Now a new paper in the Astrophysical Journal has found some hints of these.

The authors looked at archive data from the Hubble space telescope on 14 nearby galaxies. They found that in 10 of these galaxies the supermassive black hole was shifted by several light years from the optical center of the galaxy. In other words, for these galaxies the black hole was slightly off center. Since these black holes can be millions or even billions of solar masses, something pretty big must have knocked them out of the center.

It’s generally been assumed that such shifts would be due to a black hole merger, where one black hole can collide with another and cause it to shift position. But given that such a large majority of these galaxies had shifted black holes, that isn’t likely to be the case for all of them. Black hole mergers just aren’t common enough to cause all these shifted black holes.

So if mergers can’t account for all of these shifted black holes, what can? We aren’t entirely sure, but the authors did find a possible mechanism. When they looked at the direction of shift for these 1o black holes, they found they tended to be along the same direction as the jets of these black holes. This means it’s possible that asymmetrical jet emissions could cause the black holes to recoil, thus shifting them away from galactic center. We’ve seen similar effects with neutron stars, where the asymmetry of the supernova producing the neutron star causes it to recoil at high speed.

The statistical alignment observed could be caused by other mechanisms as well, so there isn’t enough evidence to confirm the jet hypothesis. What is clear is that the dynamics of supermassive black holes are more complex than once thought, and some of these effects can cause million solar-mass black holes to recoil in measurable ways, which is pretty impressive when you think about it.

Paper: D. Lena et al. Recoiling Supermassive Black Holes: A Search in the Nearby Universe. ApJ 795 146 (2014)

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One of the big mysteries in cosmology is how supermassive black holes formed in the centers of galaxies. Did they form directly from large concentrations of matter and dark matter, or did they form when early stars collided and accreted into massive black holes? Another idea is that they may have formed from the collapse of supermassive stars. In this idea stars with masses of 10,000 Suns or more could have lived short, violent lives before their core collapsed into a massive black hole. It’s an interesting idea, but new research shows that such supermassive stars might have a different fate.

This new research has been published in the Astrophysical Journal, and it looks at computer simulations of early supermassive stars. The team ran simulations of primordial (population III) stars with masses around 55,000 solar masses. At this scale, a simple hydrodynamic model doesn’t work. You need to account for the effects of general relativity as well as things like photodissociation, where the intense light of a star can break apart the nuclei of atoms. The team found that such stars can only survive for about 1.7 million years before becoming unstable.

This isn’t too surprising, but what is surprising is that for stars around this mass instead of dying as a core-collapse supernova, which would produce a massive black hole, instabilities cause the star to rip apart completely, leaving no remnant core. What’s more, a good fraction of the star’s mass has been fused into “metals” or elements beyond hydrogen and helium. As a result, such stars could provide a mechanism for the early enrichment of heavy elements in the universe.

The authors go on to point out that such a stellar explosion might be observable in the distant universe. If such stellar explosions occurred, they would be seen in the near infrared, looking similar to supernovae with high redshift (around z = 20). But because of the intense mixing of elements in the star, the explosion would look distinctly different from usual supernovae. Right now we don’t have the ability to observe such explosions, but future missions such as the Euclid infrared telescope (scheduled to be launched in 2020) might be capable.

So we haven’t solved the mystery of supermassive black holes, but we may have discovered a new way for early stars to seed the universe with heavy elements.

Paper: Ke-Jung Chen et al. The General Relativistic Instability Supernova of a Supermassive Population III Star.  ApJ 790 162 (2014)

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The image above shows two supermassive black holes orbiting each other. It is a composite image where the blue/white indicates x-rays and the pink indicates radio wavelengths. It may look like they are orbiting closely, but the black holes are about 25,000 light years apart, which is about the same distance the Sun is from the center of the Milky Way.

What’s particularly striking about this image is just how clearly we can see the features of this binary system. The hot accretion regions surrounding the black holes clearly show their locations, and each black hole shows jets in radio. It’s not often that we can see a supermassive black hole with such detail.

These two black holes are in the process of merging. Within millions of years they will eventually coalesce into a single supermassive black hole. We aren’t sure just how long that will take because we don’t know the exact masses of these black holes.

Such black hole mergers are relativity common on a cosmic scale. In about 4 billion years our own Milky Way will collide with the Andromeda galaxy, and over time the supermassive black hole in our galaxy with merge with Andromeda’s supermassive black hole.

So in a way this image hints at our future, when we shall do the gravity tango.

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A new paper in Nature has announced the discovery of a close binary of supermassive black holes. Known as J1502SE and J1502SW, the two black holes are estimated to have a mass of about 100 million solar masses each, and they are separated by only 450 light years.  This means they orbit each other once every 4 million years.  For supermassive black holes in the center of a galaxy, this is actually pretty close. So close that initially it was thought to be a single quasar. It was only through observations by the VLBI radio telescope array that the binary nature was revealed.

Radio image of the binary pair. Credit: R.P. Deane et al.

The black holes are close enough that their jets spiral in a helix pattern.  One of the black holes is active, and this spiral effect is observed in its jet. This phenomenon has been seen with binary solar mass black holes, but the fact that it can occur with supermassive black holes provides another method for their detection. For galaxies where the core is too obscured with dust, a helical jet could reveal a binary core.

Part of the reason there is an interest in such close binary systems is that they are formed by galactic mergers. By better understanding supermassive binaries, we can gain insight into the frequency and nature of galactic mergers.  Close binaries could also serve as a test for general relativity due to their high mass and close proximity.

Paper: R. P. Deane, et al. A close-pair binary in a distant triple supermassive black hole system. Nature, 2014; DOI: 10.1038/nature13454

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One of the challenges to understanding black holes is that when things get close to a black hole, things get complicated.  We actually have a good description of black holes by themselves, but the description of the heated material near a black hole is complex.  To understand the behavior of this material you need to account for not only the gravitational attraction of the black hole, but also things such as magnetic fields.  To model active black holes, you need sophisticated computer simulations, and those simulations rely on certain assumptions about how black holes interact.The assumptions we make about black holes is based upon observations we have of black holes.  Some properties, such as rotation, we’ve been able to get good measures of, but other properties such as the strength of magnetic fields near a black hole have been more challenging. Now a new paper in Nature has presented a good measure of magnetic field strength near supermassive black holes, and it is a bit surprising.

Comparison of magnetic flux vs accretion disk brightness. Credit: M. Zamaninasab, et al.

In the paper the authors looked at 76 active (radio loud) supermassive black holes. First they measured the brightness of the accretion disk of each black hole, then they measured the jets emitted from the black holes, from which they could determine the strength of their magnetic fields.   They then compared the brightness of the accretion disks with the strength of the magnetic fields.  They found the two were strongly correlated across seven orders of magnitude.

What this means is that the magnetic field plays a crucial role in the production of black hole jets across a wide range of black hole masses.  From this correlation they could also determine the strength of the magnetic field near the black hole itself. It turned out to be much stronger than expected.  So strong that it can seriously effect the behavior of the black hole accretion disk, such as compressing it magnetically.  It can even act to inhibit the infall of material into the black hole.

Basically, the magnetic fields near a black hole can be as strong as those in an MRI, and they can affect the surrounding material as strongly as the gravity of the black hole itself.  While we’ve known that magnetic fields have a significant effect on black hole dynamics, we hadn’t thought they were strong enough to seriously affect accretion rates.

So now we have a better understanding of black hole magnetic fields, and that means modeling these beasts will require a bit more animal magnetism.

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