The image above may look like a fairly normal picture of the night sky, but what you're looking at is a lot more special than just glittering stars. Each of those white dots is an active supermassive black hole.
And each of those black holes is devouring material at the heart of a galaxy millions of light-years away – that's how they could be pinpointed at all.
Released in 2021, this image contains 25,000 such dots, It's the most detailed map to date of black holes at low radio frequencies, an achievement that took years and a Europe-sized radio telescope to compile.
"This is the result of many years of work on incredibly difficult data," explained astronomer Francesco de Gasperin of the University of Hamburg in Germany back in February 2021.
"We had to invent new methods to convert the radio signals into images of the sky."
When they're just hanging out not doing much, black holes don't give off any detectable radiation, making them much harder to find.
When a black hole is actively accreting material – spooling it in from a disc of dust and gas that circles it much as water circles a drain – the intense forces involved generate radiation across multiple wavelengths that we can detect across the vastness of space.
What makes the above image so special is that it covers the ultra-low radio wavelengths, as detected by the LOw Frequency ARray (LOFAR) in Europe. This interferometric network consists of around 20,000 radio antennas, distributed throughout 52 locations across Europe.
Currently, LOFAR is the only radio telescope network capable of deep, high-resolution imaging at frequencies below 100 megahertz, offering a view of the sky like no other.
This data release, covering four percent of the northern sky, was the first for the network's ambitious plan to image the entire northern sky in ultra-low-frequencies, the LOFAR LBA Sky Survey (LoLSS).
Because it's based on Earth, LOFAR does have a significant hurdle to overcome that doesn't afflict space-based telescopes: the ionosphere.
This is particularly problematic for ultra-low-frequency radio waves, which can be reflected back into space. At frequencies below 5 megahertz, the ionosphere is opaque for this reason.
The frequencies that do penetrate the ionosphere can vary according to atmospheric conditions. To overcome this problem, the team used supercomputers running algorithms to correct for ionospheric interference every four seconds. Over the 256 hours that LOFAR stared at the sky, that's a lot of corrections.
This is what has given us such a clear view of the ultra-low-frequency sky.
"After many years of software development, it is so wonderful to see that this has now really worked out," said astronomer Huub Röttgering of Leiden Observatory in the Netherlands.
The results were published in Astronomy & Astrophysics.
A version of this article was first published in February 2021.