Welcome to my blog "Mystery of Galaxy". The Event Horizon team released the spectacular image of the black hole at M87. It's also been a little bit of an obsession of mine to really understand exactly what it is we are seeing and what we are not seeing in this image. So let's take a deep look into the abyss, to the very edge of space and time as we know it.
For starters, this black hole is located at the center of M87,a giant elliptical galaxy about 53 million light-years away in the constellation Virgo. And M87 is one of the largest galaxies in our local universe, and may have a mass as great as 200 times the Milky Way. The dominant feature of this image is a five thousand light-year jet.
This jet is accelerating away from the black hole at speeds that are a sizable fraction of light. Although the jet appears to be pointing toward the right in this image, we have to remember that it's 53 million light years away and five thousand light years long. In reality, the jet is only shifted away from our direct line-of-sight by about 17 degrees so at this distance it appears foreshortened and pointing toward the right on the sky. The jet is powered by the supermassive black hole at the center of the galaxy. The blackhole is so large that its event horizon is larger than our entire solar system.
A black hole's event horizon is a purely mathematical surface. Inside, there is nothing that can move fast enough to escape the black hole's gravity, and that includes light itself. It's the ultimate point of no return; any events that take place inside this region are forever hidden from the rest of the universe. That's why we call it the event horizon. The black hole's event horizon is defined by the Schwarzschild radius which in turn is defined by the mass of the black hole multiplied by some constants. The more massive the black hole, the larger it Schwarzschild radius and its event horizon.
The event horizon represents the minimum distance that light can escape from it, but only if it's traveling directly away from the black hole. If light is moving at some sideways direction, its path is bent until it falls back into the black hole. At 1.5 times a Schwarzschild radius, light is trapped into an orbit. This is called the photon sphere. But this orbit is unstable. Light either falls into the black hole or escapes, but even then it is bent by the black hole's gravity. This effect is called gravitational lensing, if the black hole is rotating, light can orbit at a range of distances from the event horizon so it can get lensed out to a distance of about 2.5 times the Schwarzschild radius. The result is a shadow about five times the event horizon's diameter cast upon a ring of lensed photons. And that means we can never actually see M87's event horizon or even its photon sphere. In the best-case scenario, we can only see its shadow and the lensed photon ring.
So where are these photons coming from?
Well, there are two sources. The first is from the accretion disk that surrounds the black hole. Matter spirals around the black hole until it reaches a critical distance where it can no longer maintain a stable circular orbit. This inner most stable circular orbit is called ISCO and represents the point of no return for any matter that gets any closer. Depending on the black hole spin, the disc cuts off at about three times the Schwarzschild radius. Any closer and matter spirals in. Matter in this region is heated to billions of Kelvin and radiates mostly at x-ray wavelengths, This is not the light that we are seeing in the EHT image. But some of this matter is accelerated into a vortex which in turn powers the 5000 light-year jet. A powerful magnetic field forms inside the vortex. Free electrons spiral around the magnetic field lines emitting synchrotron radiation. These synchrotron photons are then caught up in the black hole's photon sphere before being lensed in our direction. These photons have a characteristic wavelength of around a millimeter which is long enough for them to pass through the surrounding maelstrom and produce the image that we are seeing. Obviously, the image appears a little blurry but at the same time it's still the highest resolution image we've ever taken. To understand why let's remember how hard it is to actually create an image in the first place. It all has to do with angular resolution. To quickly recap, a telescope's angular resolution depends on the wave length it's observing and its overall diameter. In order to resolve the photon ring at1.3 millimeters, the telescope needs to be the size of Earth. EHT achieves an astonishing 25 micro arc second resolution. That's sharp enough to read the lettering on a dime in Los Angeles from New York City. M87's photon ring is 42 micro arc second across so EHT is just able to distinguish it. But the image is a little bit blurred and that's why the ring appears thicker than it probably is in real life. It also helps explain why the shadow isn't completely dark; some of the photon ring's light is being blurred into it. But there's another reason why the image appears the way it does: some of that light is coming from plasma funneling its way into the black hole as well as being launched in the jet. Now exactly what we see depends in part on our viewing angle but the any emission that originates in front of the black hole proceeds directly toward us while emission from behind the blackhole is lensed around and adds to the photon ring. The most obvious feature of this image is that the ring appears asymmetric and that's due to the black hole's spin. Any plasma that is rotating toward us has its emission Doppler boosted while any emission that is rotating away from us is dimmed by the exact same effect.
But exactly how is the black hole spinning?
To find out the team modeled how the black hole might appear using a series of general relativistic magneto hydrodynamic simulations. These simulations model the dynamics of the magnetic field, fluid flows from the disk and jet, as well as the warped space-time due to the gravity of the black hole. Since the jet is tilted by about 17 degrees from our point of view, the team was able to use that to constrain the rotational axis of the black hole. Now all they had to do is work out different parameters for the mass, the spin direction, the spin rate, electron temperature, and a host of other parameters. They found that a clockwise spinning black hole produced a brightening on the ring at the bottom of the image. Those results were then blurred to match EHT's resolution and then compared to the actual image. The best match corresponds to a black hole that is six and a half billion times the mass of our Sun rotating clockwise on the sky. That means M87's Schwarzschild radius is 127 astronomical units. If our Sun were at the center of this image, Pluto's orbit would fit well inside this black hole's event horizon. In fact, Voyager 1 would have flown just beyond the black hole's event horizon except for the fact that it's unable to because of the whole speed of light thing.
There's also some variability in the rings. The April 5 and 6 images show the asymmetry oriented toward the seven o'clock position while the April 10 and 11images seem to be oriented toward the six o'clock position. It's not yet clear what is causing this asymmetry but M87 is approximately one light-day across, so that would set a minimum amount of time for events to transpire across this black hole's event horizon. Now this is just the first image made by EHT, but they would like to improve it and there's a couple of ways to do that. Remember, this image is a reconstruction of several partial images made by telescopes around the planet. Think of it as a mirror with tiny pieces of glass separated or a great distance. Each piece of glass captures a portion of the true image. By adding more telescopes, astronomers will be able to improve the fidelity of the image - that is, get an image that's that much closer to the truth, and has to rely less on algorithms to reconstruct the rest. To that end, EHT already added the Greenland telescope in 2018 and they're in the process of adding the NOEMA array at the IRAM Observatory in the French Alps, as well as the 12-meter telescope at Kitt Peak National Observatory in Arizona. Second, EHT plans to observe at shorter wavelengths, going from one point three millimeter down to 0.87 millimeter. Now that may not seem like a significant reduction in wavelength, but remember angular resolution is directly proportional to the wavelength, so even by going down just by a little bit, they'll be able to improve the resolution of their image by about 30 percent.
Another way to improve the image would be to increase the baseline. That is, increase the space between the telescopes. Since the EHT telescope is already the size of Earth. the next logical step would be to deploy radio telescopes in space. Even having just one radio receiver in low-earth orbit would dramatically improve the resolution of EHT. And then there's the black hole at the center of our galaxy, Sagittarius A-star. Sag A-star is about 26,000 light years away, which means it's a whole lot closer than M87. But Sag A-star is also a thousand times less massive, which means it's a thousand times smaller. That makes our galaxy's black hole a lot more dynamic and harder for EHT to pin down in a single image. However, the team have data that they acquired on Sag A* in 2017 and again in 2018, and they're presently in the process of teasing out a real image from all of that data. Now making these images is hard and there's a lot of work that goes into them. We are in a new era of astronomy. We've now gazed into the abyss of a black hole reaching all the way to the very farthest edge of space and time as we know it. There is much much more to learn and I look forward to sharing that with you as we find out more. Thank you for reading.
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| M87 blackhole source: Event Horizon Telescope / CC BY |
For starters, this black hole is located at the center of M87,a giant elliptical galaxy about 53 million light-years away in the constellation Virgo. And M87 is one of the largest galaxies in our local universe, and may have a mass as great as 200 times the Milky Way. The dominant feature of this image is a five thousand light-year jet.
 |
| Jets from super massive black hole Source: ESA/Hubble / CC BY |
This jet is accelerating away from the black hole at speeds that are a sizable fraction of light. Although the jet appears to be pointing toward the right in this image, we have to remember that it's 53 million light years away and five thousand light years long. In reality, the jet is only shifted away from our direct line-of-sight by about 17 degrees so at this distance it appears foreshortened and pointing toward the right on the sky. The jet is powered by the supermassive black hole at the center of the galaxy. The blackhole is so large that its event horizon is larger than our entire solar system.
 |
| Event horizon Source: Deutsch: Black_Hole_Milkyway.jpg: Ute Kraus, Physikdidaktik Ute Kraus, Universität Hildesheim, Tempolimit Lichtgeschwindigkeit, (Milchstraßenpanorama im Hintergrund: Axel Mellinger)abgeleitetes Werk: Sponk (talk)English: Black_Hole_Milkyway.jpg: Ute Kraus, Physics education group Kraus, Universität Hildesheim, Space Time Travel, (background image of the milky way: Axel Mellinger)derivative work: Sponk / CC BY-SA 2.0 DE (https://creativecommons.org/licenses/by-sa/2.0/de/deed.en) |
A black hole's event horizon is a purely mathematical surface. Inside, there is nothing that can move fast enough to escape the black hole's gravity, and that includes light itself. It's the ultimate point of no return; any events that take place inside this region are forever hidden from the rest of the universe. That's why we call it the event horizon. The black hole's event horizon is defined by the Schwarzschild radius which in turn is defined by the mass of the black hole multiplied by some constants. The more massive the black hole, the larger it Schwarzschild radius and its event horizon.
The event horizon represents the minimum distance that light can escape from it, but only if it's traveling directly away from the black hole. If light is moving at some sideways direction, its path is bent until it falls back into the black hole. At 1.5 times a Schwarzschild radius, light is trapped into an orbit. This is called the photon sphere. But this orbit is unstable. Light either falls into the black hole or escapes, but even then it is bent by the black hole's gravity. This effect is called gravitational lensing, if the black hole is rotating, light can orbit at a range of distances from the event horizon so it can get lensed out to a distance of about 2.5 times the Schwarzschild radius. The result is a shadow about five times the event horizon's diameter cast upon a ring of lensed photons. And that means we can never actually see M87's event horizon or even its photon sphere. In the best-case scenario, we can only see its shadow and the lensed photon ring.
 |
| Source: NASA’s Goddard Space Flight Center/Jeremy Schnittman / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) |
So where are these photons coming from?
Well, there are two sources. The first is from the accretion disk that surrounds the black hole. Matter spirals around the black hole until it reaches a critical distance where it can no longer maintain a stable circular orbit. This inner most stable circular orbit is called ISCO and represents the point of no return for any matter that gets any closer. Depending on the black hole spin, the disc cuts off at about three times the Schwarzschild radius. Any closer and matter spirals in. Matter in this region is heated to billions of Kelvin and radiates mostly at x-ray wavelengths, This is not the light that we are seeing in the EHT image. But some of this matter is accelerated into a vortex which in turn powers the 5000 light-year jet. A powerful magnetic field forms inside the vortex. Free electrons spiral around the magnetic field lines emitting synchrotron radiation. These synchrotron photons are then caught up in the black hole's photon sphere before being lensed in our direction. These photons have a characteristic wavelength of around a millimeter which is long enough for them to pass through the surrounding maelstrom and produce the image that we are seeing. Obviously, the image appears a little blurry but at the same time it's still the highest resolution image we've ever taken. To understand why let's remember how hard it is to actually create an image in the first place. It all has to do with angular resolution. To quickly recap, a telescope's angular resolution depends on the wave length it's observing and its overall diameter. In order to resolve the photon ring at1.3 millimeters, the telescope needs to be the size of Earth. EHT achieves an astonishing 25 micro arc second resolution. That's sharp enough to read the lettering on a dime in Los Angeles from New York City. M87's photon ring is 42 micro arc second across so EHT is just able to distinguish it. But the image is a little bit blurred and that's why the ring appears thicker than it probably is in real life. It also helps explain why the shadow isn't completely dark; some of the photon ring's light is being blurred into it. But there's another reason why the image appears the way it does: some of that light is coming from plasma funneling its way into the black hole as well as being launched in the jet. Now exactly what we see depends in part on our viewing angle but the any emission that originates in front of the black hole proceeds directly toward us while emission from behind the blackhole is lensed around and adds to the photon ring. The most obvious feature of this image is that the ring appears asymmetric and that's due to the black hole's spin. Any plasma that is rotating toward us has its emission Doppler boosted while any emission that is rotating away from us is dimmed by the exact same effect.
But exactly how is the black hole spinning?
To find out the team modeled how the black hole might appear using a series of general relativistic magneto hydrodynamic simulations. These simulations model the dynamics of the magnetic field, fluid flows from the disk and jet, as well as the warped space-time due to the gravity of the black hole. Since the jet is tilted by about 17 degrees from our point of view, the team was able to use that to constrain the rotational axis of the black hole. Now all they had to do is work out different parameters for the mass, the spin direction, the spin rate, electron temperature, and a host of other parameters. They found that a clockwise spinning black hole produced a brightening on the ring at the bottom of the image. Those results were then blurred to match EHT's resolution and then compared to the actual image. The best match corresponds to a black hole that is six and a half billion times the mass of our Sun rotating clockwise on the sky. That means M87's Schwarzschild radius is 127 astronomical units. If our Sun were at the center of this image, Pluto's orbit would fit well inside this black hole's event horizon. In fact, Voyager 1 would have flown just beyond the black hole's event horizon except for the fact that it's unable to because of the whole speed of light thing.
 |
| Voyager 1 Source NASA, ESA, and G. Bacon (STScI) / CC BY (https://creativecommons.org/licenses/by/4.0) |
There's also some variability in the rings. The April 5 and 6 images show the asymmetry oriented toward the seven o'clock position while the April 10 and 11images seem to be oriented toward the six o'clock position. It's not yet clear what is causing this asymmetry but M87 is approximately one light-day across, so that would set a minimum amount of time for events to transpire across this black hole's event horizon. Now this is just the first image made by EHT, but they would like to improve it and there's a couple of ways to do that. Remember, this image is a reconstruction of several partial images made by telescopes around the planet. Think of it as a mirror with tiny pieces of glass separated or a great distance. Each piece of glass captures a portion of the true image. By adding more telescopes, astronomers will be able to improve the fidelity of the image - that is, get an image that's that much closer to the truth, and has to rely less on algorithms to reconstruct the rest. To that end, EHT already added the Greenland telescope in 2018 and they're in the process of adding the NOEMA array at the IRAM Observatory in the French Alps, as well as the 12-meter telescope at Kitt Peak National Observatory in Arizona. Second, EHT plans to observe at shorter wavelengths, going from one point three millimeter down to 0.87 millimeter. Now that may not seem like a significant reduction in wavelength, but remember angular resolution is directly proportional to the wavelength, so even by going down just by a little bit, they'll be able to improve the resolution of their image by about 30 percent.
Another way to improve the image would be to increase the baseline. That is, increase the space between the telescopes. Since the EHT telescope is already the size of Earth. the next logical step would be to deploy radio telescopes in space. Even having just one radio receiver in low-earth orbit would dramatically improve the resolution of EHT. And then there's the black hole at the center of our galaxy, Sagittarius A-star. Sag A-star is about 26,000 light years away, which means it's a whole lot closer than M87. But Sag A-star is also a thousand times less massive, which means it's a thousand times smaller. That makes our galaxy's black hole a lot more dynamic and harder for EHT to pin down in a single image. However, the team have data that they acquired on Sag A* in 2017 and again in 2018, and they're presently in the process of teasing out a real image from all of that data. Now making these images is hard and there's a lot of work that goes into them. We are in a new era of astronomy. We've now gazed into the abyss of a black hole reaching all the way to the very farthest edge of space and time as we know it. There is much much more to learn and I look forward to sharing that with you as we find out more. Thank you for reading.
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