The image above may look like a fairly normal image of the night sky, but what you look at is much more special than just the twinkling stars. Each of these white dots is an active supermassive black hole.
And each of these black holes devours material in the heart of a galaxy millions of light-years away – so it could not be identified at all.
In total, 25,000 such points, astronomers have created the most detailed map of black holes at low radio frequencies so far, an achievement that took years and a European-sized radio telescope to compile.
“This is the result of many years of work on incredibly difficult data,” said astronomer Francesco de Gasperin of the University of Hamburg in Germany. “We had to invent new methods to convert radio signals into images of the sky.”
(LOFAR / LOL Surveys)
When they don’t stay too long, black holes do not emit detectable radiation, which makes them much harder to find. When a black hole actively increases the material – scattering it from a disk of dust and gas that surrounds it as water surrounds a drain – the intense forces involved generate radiation at multiple wavelengths that we can detect in the vastness of space.
What makes the image above so special is that it covers very short radio wavelengths, as detected by the LOw Frequency ARray (LOFAR) in Europe. This interferometric network consists of approximately 20,000 radio antennas, distributed in 52 locations in Europe.
Currently, LOFAR is the only radio telescope network capable of producing high-resolution images at frequencies below 100 megahertz, providing a view of the sky like no other. This data communication, which covers four percent of the northern sky, is the first for the network’s ambitious plan to imagine the entire northern sky in ultra-low frequencies, the LOFAR LBA Sky Survey (LoLSS).
Because it is based on Earth, LOFAR has a significant obstacle to overcome that does not affect space telescopes: the ionosphere. This is especially 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.
Frequencies entering the ionosphere may vary depending on weather conditions. To overcome this problem, the team used supercomputers running algorithms to correct ionospheric interference every four seconds. In the 256 hours that LOFAR has looked at the sky, this is a lot of corrections.
This is what gave us such a clear view of the sky with ultra-low frequency.
“After many years of software development, it’s so wonderful to see that this really worked,” said astronomer Huub Röttgering of the Leiden Observatory in the Netherlands.
Having to correct the ionosphere has another benefit: it will allow astronomers to use LoLSS data to study the ionosphere itself. Ionospheric traveling waves, scintillations, and the relationship of the ionosphere to solar cycles could be characterized in much greater detail by LoLSS. This will allow scientists to better constrain ionospheric patterns.
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 launch of the survey will facilitate progress in a number of areas of astronomical research,” the researchers wrote in their paper.
“[This] will allow the study of over 1 million low-frequency radio spectra, providing unique information on physical models for galaxies, active nuclei, galaxy clusters and other areas of research. This experiment is a unique attempt to explore the sky with ultra-low frequency at a high resolution and angular depth. “
The results are to be published in Astronomy and astrophysics.