New images of the universe’s most photogenic eclipse provide insight into the behavior of a mysterious black hole.
For the first time, we’re looking at the source of a colossal jet of plasma being blasted into space from the rim of supermassive black hole M87*. It’s also the first time we’ve seen a black hole’s shadow and its jet together in the same image, a view that should help astronomers figure out how these giant jets of plasma are produced.
“We know that jets are ejected from around black holes,” says astronomer Ru-Sen Lu of the Shanghai Astronomical Observatory in China, “but we still don’t fully understand how this actually happens. To study this directly, we need to observe the origin of the jet as close to the black hole as possible.”
As we all know, black holes are notorious for not emitting anything we can detect. They are so dense that space-time effectively warps into a closed sphere around them, leaving no speed in the universe high enough to achieve escape velocity. But the space just outside the boundary of this ball – what we call the event horizon – is a different matter.
Here is a region of extremes where gravity reigns supreme. Any nearby material is caught in its snare and swirls into a disk of material that flows toward the black hole like water down a drain. Friction and gravity heat this material and cause it to glow; We saw that in the now-famous image of M87*, first released in 2019, from data collected in 2017 by the Event Horizon Telescope (EHT) collaboration.
But not all material is necessarily pulled beyond the event horizon. Some of it skims the rim before being ejected into space by the black hole’s polar regions, forming jets that travel at a significant percentage of the speed of light and can punch great distances into interstellar space.
Astronomers believe this material is deflected along magnetic field lines outside the event horizon by the inner edge of the disk. These magnetic field lines accelerate the particles so that when they reach the poles, they are thrown into space at great speed.
Those are the big lines; the details are harder to pin down. We know that M87* has a jet reaching 100,000 light-years at radio wavelengths, about the diameter of our own galaxy. So in 2018, astronomers used powerful radio telescopes, combined into the Global mm-VLBI Array (GMVA), to see if they could map the region from which the jets launch in detail. It collected data at a longer wavelength than the EHT and revealed different information.
“M87 has been observed for many decades, and 100 years ago we knew the jet was there, but we couldn’t put it in context,” says Lu. “With GMVA, including the world-class instruments at NRAO and GBO, we’re observing at a lower frequency so we’re seeing more detail — and now we know there’s more detail to see.”
About 55 million light-years distant, galaxy M87 is home to a supermassive black hole about 6.5 billion times the mass of our Sun that is actively accumulating matter from a disk around it. The image captured by the EHT showed for the first time the shadow of this black hole – a dark region in the middle of a glowing ring of material, distorted by the gravitational curvature of spacetime.
The new image shows a wider area of space than the EHT image. It shows that the extent of the plasma around M87* is much larger than what we see in the EHT image, in addition to the source of the jet.
“The original EHT imaging showed only part of the accretion disk surrounding the black hole’s center. By changing the observation wavelength from 1.3 millimeters to 3.5 millimeters, we can see more of the accretion disk and now the jet at the same time,” says astronomer Toney Minter of the National Radio Astronomy Observatory. “It showed that the ring around the black hole is 50 percent larger than previously thought.”
The new image also revealed new information about how the jet is launched from the region of space around the black hole, confirming that magnetic field lines do indeed play a crucial role in sweeping away material intended to be launched as jets.
But they don’t act alone. A strong wind emanates from the disk itself, driven by radiation pressure. As the image shows, this wind contributes to the formation of the M87 jet.
This is a fairly significant breakthrough in black hole science, but researchers aren’t done yet. There’s a lot more to see across the radio spectrum, and M87* has proven it can do it.
“We plan to observe the region around the black hole at the center of M87 at different radio wavelengths to further study the emission from the jet,” says astronomer Eduardo Ros from the Max Planck Institute for Radio Astronomy in Germany. “The years to come will be exciting as we learn more about what is happening near one of the most mysterious regions in the universe.”
The research was published in Nature.
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