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 25,000 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.
In a new study, astronomers created 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," Francesco de Gasperin, an astronomer from the University of Hamburg and co-author of the study, said in a press release. "We had to invent new methods to convert the radio signals into images of the sky."
A high-resolution map of the Northern sky
When they're just hanging out not doing much, black holes don't give off any detectable radiation, making them hard 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 network consists of around 20,000 radio antennas, distributed throughout 52 locations across nine European countries.
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, covering 4% of the northern sky, is the first for the network's ambitious plan to image the entire northern sky in ultra-low frequencies, called 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," Huub Röttgering, a study co-author and an astronomer from the Netherlands' Leiden Observatory, said in the release.
Having to correct for the ionosphere has another benefit, too — it will allow astronomers to use LoLSS data to study the ionosphere itself. Ionospheric traveling waves, scintillations, and the relationship of the ionosphere with solar cycles could be characterized in much greater detail with the LoLSS. This will allow scientists to better constrain ionospheric models.
And the survey will provide new data on all sorts of astronomical objects and phenomena, as well as possibly undiscovered or unexplored objects in the region below 50 megahertz.
"The final release of the survey will facilitate advances across a range of astronomical research areas," the study authors wrote, adding that this "will allow for the study of more than 1 million low-frequency radio spectra, providing unique insights on physical models for galaxies, active nuclei, galaxy clusters, and other fields of research. This experiment represents a unique attempt to explore the ultra-low frequency sky at a high angular resolution and depth."
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