Scientists have succeeded in fitting this image of the Sagittarius-A-star black hole based on data collected from eight radio observatories distributed around the world.
Credit: Event Horizon Telescope collaboration
Radio astronomers have captured an image of the supermassive black hole at the center of the Milky Way. This is the second time that a direct image of a black hole has been taken, after the first – and historical – image that the same team was able to capture of a black hole much farther away, which was unveiled in 2019.
The long-awaited results, which were published by the Event Horizon Telescope collaborative on May 12, show an image reminiscent of its predecessor: a ring of radiation surrounding a dark disk of exactly the same size as predicted by indirect observations and general relativity. Developed by Albert Einstein.
“Today, at this moment, we have direct evidence that this object is a black hole,” Sarah Esson, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics based in Cambridge, Massachusetts, said at a press conference in Garching, Germany. The team published its findings in a special issue of The Astrophysical Journal Letters. The Astrophysical Journal Letters. (K. Akiyama et al.
Astrophys. J. Lett. 930L12; 2022.)
“We’ve been preparing for this discovery for a long time,” Katie Bowman, a former Event Horizon Telescope team member and a computational imaging researcher at the California Institute of Technology in Pasadena, told a news conference in Washington. It is the black hole at the heart of our galaxy.
Planet-sized telescope
Over the course of five nights in April 2017, the Event Horizon Telescope Collaboration used eight observatories around the world to collect data from the Milky Way’s black hole — which scientists named Sagittarius A* after The constellation in which it was discovered – as well as from the black hole M87*, which is in the center of the galaxy M87.
The locations of the observatories were distributed over various regions, from Spain to Antarctica, and from Chile to Hawaii (see:
global effort).
Researchers unveiled their 2019 image of the M87* black hole, providing the first direct evidence of an “event horizon”, the spherical surface that surrounds a black hole’s interior.
But analyzing the Sagittarius A Star data was more difficult. The two black holes have about the same apparent size in the sky; It is true that the black hole M87* is about 1,600 times larger than Sagittarius-A star, but it is about 2,000 times further away. In addition, the clumps of material orbiting around the M87* black hole – whatever their nature – travel much greater distances (larger than Pluto’s orbit around the Sun), and any radiation they emit is practically constant over a short timescale, unlike a black hole Sagittarius A star, which could change rapidly, even within the few hours that the EHT monitors every day.
“In the case of the M87* black hole, a whole week passes and we see very little change,” says astrophysicist Heinau Valcke of Radboud University in Nijmegen, the Netherlands, and co-founder of the Event Horizon Telescope collaboration. It changes shape within 5 to 15 minutes.”
Because the shape of Sagittarius-a-star changes so quickly, scientists have been careful not to produce a single image of it, but thousands of images. “By averaging the data from all these images, we can check for common features,” said Jose Gomez, a researcher at the Andalusian Institute for Astrophysics in Granada, Spain, and one of the team.
In addition to the radiation ring surrounding a dark disk, the resulting image also showed three bright “nodes”. “We see knots in all the photos we produce,” says Asaun, but their location is different in each photo. She added that these knots likely represent defects in the interferometry method used by the researchers.
This black hole differs in its shape from the hole M87*, whose bright region was in the shape of a lunar quadrature (half full moon), which may indicate the presence of denser clumps accelerating, along the line of sight.
Using a supercomputer, the team ran simulations, comparing their data with their results, and concluded that Sagittarius-A-star most likely rotates counterclockwise, about an axis that points almost as far as line-of-sight toward Earth, Gomez says.
“What really amazes me is that we see it from the opposite direction,” says Regina Caputo, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The Fermi Gamma-ray Space Telescope, also of NASA and which Kabuto works with, had previously observed gigantic flarings above and below the galactic center not far from being generated by Kabuto. The Sagittarius-A-Star wormhole occurred during periods of intense activity in the past. But these shapes, called Fermi bubbles, seem to require that matter swirls around the black hole in swirls around the black hole when viewed from the sides, not from the front as seen from Earth.
The first indications of the existence of the black hole “Sagittarius-A-star” appeared in the 1970s, when radio astronomers discovered a source of radio waves, similar to a point located in the center of the galaxy.
This source turned out to be unusually opaque; It’s too dark to be an ordinary star. Nevertheless, decades of observations of the movement of neighboring stars have proven that this object is extremely massive: based on the latest observed results, scientists were able to calculate that its mass, which is 4.15 million solar masses, with a margin of error of plus or minus 0.3% . These calculations, made by tracking the way stars orbit Sagittarius-A-star, provide conclusive evidence that the source of these radio waves is so massive and dense that it can only be a black hole.
It is worth noting that the black hole “Sagittarius-A star” is practically invisible to optical telescopes; Due to dust and gas covering the galactic disk. But starting in the late 1990s, Falk and others realized that a black hole’s shadow might be large enough to be imaged by the short radio waves that can escape from that envelope. However, scientists estimated that it would require a telescope the size of Earth. Fortunately, a technique called interferometry can help scientists in this endeavour. The technology relies on pointing several remote telescopes at a single object at the same time, making the telescopes practically as if they were pieces of one large dish.
The first attempts to observe “Sagittarius-A-star” by interferometry relied on radio waves with a wavelength of 7 millimeters, which is relatively long, and the observatories were several thousand kilometers away from each other. Thus, the researchers could not help but see a cloudy spot.
Then various teams around the world worked hard to improve their methods, and re-equipped observatories that were added to the network. Specifically, the researchers put the Antarctic Telescope and the $1.4 billion Atacama Large Millimeter/Submillimeter Telescope Array in Chile to do just that.
In 2015, research groups joined forces to form the Event Horizon Telescope Collaboration. These groups participated in a joint space-observing campaign in 2017, which was the first to include observatories at a sufficient distance to reveal details of an object the size of a Sagittarius A star.
future plans
The Event Horizon Telescope team collected more data in 2018, but canceled observations scheduled for 2019 and 2020. Then monitoring resumed in 2021 and 2022, with an improved network and more sophisticated equipment.
Remo Telanos, a team member at the University of Arizona in Tucson, says that the team’s last observations, in March, recorded signals at twice the rate of those in 2017, mostly at a wavelength of 0.87 millimeters, which may contribute to the higher resolution of the resulting images.
Researchers hope to discover whether the Sagittarius-A-star black hole has vents; Many black holes, including M87*, appear to have two rays of matter spewing out rapidly in opposite directions, and it is believed that they are caused by the intense heating of the gas that falls towards the black hole, and is pushed by its spin. Sagittarius-A-Star may have had large jets in the past, indicated by clouds of heated matter above and below the galactic center. Its jets will now be much weaker, but their presence could reveal important details about the history of our galaxy.
“These bursts can dampen or stimulate star formation, and they can push chemical elements around,” Falk says, affecting the evolution of an entire galaxy. “We are now looking at where all of this is happening,” he added.