An international team of astronomers has released an extraordinary image and map of the sky where there is no single star, but instead filled with 25,000 supermassive black holes.
If black holes do not emit light, you may be questioning how this is possible. Although this information is partially correct and the black hole itself is essentially invisible, as it “eats” a star or other object, the tidal forces of gravity it creates create a flattened accretion disk around it as the other object is torn apart.
The material in the disk spins around the black hole’s event horizon at very close to the speed of light, and sometimes this material is ejected by the black hole’s intense magnetic field, creating jets of particles at the black hole’s magnetic poles.
These relativistic jets emit a lot of radio waves. This is also based on the mapping of the northern skies that astronomers have carried out across Europe using a network of 52 radio telescopes known as the Low Frequency Array (LOFAR).
“This is the result of many years of work on incredibly difficult data,” said lead researcher Francesco de Gasperin, formerly at Leiden University but now at the University of Hamburg, Germany: “Inventing new methods for converting radio signals into images of the sky. We had to.”
The incredible map shows low-frequency radio signals from 25,000 supermassive black holes at the center of distant galaxies and covers only 4% of the northern hemisphere’s night sky. Researchers hope to eventually create a sky atlas of supermassive black holes visible in the northern hemisphere.
LOFAR/LOL SurveyOne of the difficulties with seeing radio waves from supermassive black holes is that they often produce low-frequency radio waves. Normally this wouldn’t be a problem, but for Earth-based radio telescopes it does pose a problem as Earth’s ionosphere completely reflects radio signals below 3MHz and distorts signals up to 30MHz.
“It’s like trying to see the world while immersed in a swimming pool,” says study co-author Reinout van Weeren, of the Leiden Observatory. “When you look up, waves in the pool’s water deflect light rays and distort the image.”
To counter this, the researchers developed a supercomputer algorithm that corrected the disruptive effects of the ionosphere every four seconds of 256 hours of observation.
“After years of software development, it’s great to see this actually work,” says Huub Röttgering, Scientific Director of the Leiden Observatory, and co-author of the study.
This isn’t the first time a computer algorithm has helped astronomers map the skies. In one of the most famous examples, in 2019 Katie Bouman designed the algorithm that combined various data streams from the Event Horizon Telescope to give us the first image of a black hole’s event horizon.
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