The first image ever taken of a black hole’s shadow was recently sharpened by AI. That made for a pretty picture, but the bigger story was still to come. Refinements of images taken five years ago have now, for the first time, allowed us to see the jets expelled by the black hole at the same time as the shadow itself.
Although black holes are such powerful warpers of space-time that even light can’t escape, they are far from the stealthy threats imagined just a few decades ago. Instead, the supermassive black holes at the heart of galaxies are surrounded by immense accretion disks of swirling material from stars they have dismembered. They also create colossal jets perpendicular to the disk, accelerating charged particles to close to the speed of light, their motions emitting high-energy photons so bright they can outshine the entire galaxy from which they come.
Artists have been rendering images of black holes for years highlighting these features, such as the one below, but capturing the real thing is a different matter. However, by combining observations from the Global Millimetre VLBI Array, the Atacama Large Millimeter/submillimeter Array, and the Greenland Telescope, this is just what astronomers have done.
Although the existence of jets like these has been known for decades, the physics that produces them is still poorly understood, and the international team that has published this first-of-it's-kind image hopes their work will address that.
“We know that jets are ejected from the region surrounding black holes,” said Dr Ru-Sen Lu from the Shanghai Astronomical Observatory in a statement, “but we still do not fully understand how this actually happens. To study this directly we need to observe the origin of the jet as close as possible to the black hole.”
The authors used 3.5-millimeter radio waves to see the jet and shadow together, rather than the 1.3 mm waves used for the original shadow image. Besides revealing the jet, the longer wavelengths expose a ring around 50 percent bigger than that seen at 1.3 mm. It’s like having seen an object only in blue light, and then observing it in red and discovering there is a lot more to it.
The telescopes used for this project are based in North America and Europe, South America, and Greenland, respectively. The distance between the antennae allows resolution equal to if the entire planet was a radio dish.
It may look messy to the untrained eye, but astronomers see the base of the jet connecting to the accretion disk around M87*, the photogenic supermassive black hole at the heart of the Messier 87 galaxy 55 million light-years away. The image reveals two bright regions, associated with the jets pointing in opposite directions. “Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of wind associated with the accretion flow,” the paper notes.
M87* was chosen for this giant collaboration, as well as being the subject of that first famous image, because it is both close (by intergalactic standards) and very large, with 6.5 billion times the mass of the Sun. Sagittarius A*, the second black hole to be imaged, is much closer, being at the center of our own galaxy, but has a mass a thousand times less, as well as being hidden by a lot of dust.
Future projects will again combine telescopes from different continents to study M87* at other wavelengths. “The coming years will be exciting, as we will be able to learn more about what happens near one of the most mysterious regions in the Universe,” said Professor Eduardo Ros of the Max Plank Institute die Radio Astronomy.
The study is published in Nature.