At first glance, this glittering array of white dots against a black background looks like any other night sky. In reality, this image captures something much cooler— those starry white spots are actually thousands of supermassive black holes captured via radio signals. It’s the most detailed map of black holes—over 25,000 of them—ever produced to date.
Each of these black holes is swallowing dust and gas at the center of their own galaxy, millions of light-years away. These intense, hungry spirals create enough radiation, on multiple wavelengths, to reach us here on Earth. If these black holes weren’t actively devouring space particles, they would be nearly impossible to pinpoint because dormant black holes don’t give off any detectable radiation, rendering them more or less invisible.
Even in the midst of a cosmic meal capturing an active black hole, let alone thousands of them, is no easy task. The black hole’s ultra-low radio wavelengths were detected by the Low Frequency Array (LOFAR), a network of 20,000 radio antennas scattered across 52 locations in Europe.
LOFAR is currently the only radio telescope network with the capability to produce high-resolution images at frequencies below 100 megahertz. As a radio wave travels across space, its frequency decreases, and so does its energy. Since signals become harder to detect at lower energies, and the energy of a wave decreases as it travels millions of light-years across space, highly-sensitive arrays like LOFAR are key to detecting the energy signatures of black holes and other phenomena across space.
Once the signals are picked up by the network of antennas, the data can be compiled into images like the one above. In this particular case, researchers compiled the radio wave data collected from each antenna to estimate the location of each black hole. “This is the result of many years of work on incredibly difficult data,” explains astronomer Francesco de Gasperin of the University of Hamburg in Germany. “We had to invent new methods to convert the radio signals into images of the sky.”
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LOFAR faces a significant obstacle avoided by space-based telescopes: the ionosphere. This outermost layer of the atmosphere forms a shell of electrons and charged particles around the Earth. Stretching from an altitude of 50 kilometers above the surface to an altitude of 1,000 kilometers, it’s where the atmosphere meets the vacuum of space.
Ultra-low frequency radio waves, below 5 megahertz, are reflected back into space when they hit this atmospheric layer en route to LOFAR’s antennas on the Earth’s surface. To correct for ionospheric interference, the team of astronomers developed algorithms run by supercomputers to filter out interference every four seconds while collecting radio emissions.
“After many years of software development, it is so wonderful to see that this has now really worked out,” says astronomer Huub Röttgering of Leiden Observatory in the Netherlands.
This first image, which covers four percent of the Northern sky, combines a decade of development and analysis, and nearly 11 days worth of radio emissions absorbed by LOFAR. Researchers plan to image the entire Northern sky via ultra-low frequencies as part of the LOFAR LBA Sky Survey.
The full results of this experiment are due to be published in the journal Astronomy and Astrophysics.