Incredible sky map doesn't show stars – but thousands of black holes

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Watch: This Astonishing Image of Space Shows 25,000 Supermassive Black Holes

A lot of galaxies have "supermassive" black holes – huge monsters with masses up to 10 billion times that of our Sun – at their centre.

Now an international team of astronomers has mapped the location of 25,000 supermassive black holes, in a map which looks eerily like stars in the sky.

But each "dot" isn’t a star, it’s a black hole located in a different, distant galaxy – detected by radio emissions as matter that gets too close to the black hole is hurled back out.

The map was constructed using long radio wavelengths, and could offer new insights into supermassive black holes – and into the large-scale structure of the universe.

The research has been accepted for publication in Astronomy & Astrophysics

Read more: Astronomers find closest black hole to Earth

It’s the most detailed celestial map using "low radio frequencies" and was produced by astornomers using 52 stations with low-frequency array (LOFAR) antennas spread across nine European countries.

Research leader Francesco de Gasperin (formerly of Leiden University, now Universität Hamburg, Germany) said: "This is the result of many years of work on incredibly difficult data.

“We had to invent new methods to convert the radio signals into images of the sky.”

Each white dot is a supermassive black hole (University of Leiden)
Each white dot is a supermassive black hole. (University of Leiden)

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Watching the sky at long radio wavelengths is made more difficult by the ionosphere that surrounds the Earth.

The researchers say the layer of free electrons acts like a cloudy lens that constantly moves across a telescope.

Co-author Reinout van Weeren of Leiden Observatory says, “It's similar to when you try to see the world while immersed in a swimming pool. When you look up, the waves on the water of the pool deflect the light rays and distort the view.”

Read more: What are fast radio bursts, and why do they look like aliens?

The new map was created by combining 256 hours of observations of the northern sky, using supercomputers with new algorithms that correct the effect of the ionosphere every four seconds.

Scientific director of the Leiden Observatory Huub Röttgering said: “After many years of software development, it is so wonderful to see that this has now really worked out."

The map now covers 4% of the northern half of the sky – and the astronomers plan to continue until they have mapped the entire northern sky.

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