While the image on the cover resembles an image you’ll see when you look up at the sky on a cloudless night away from light pollution, you’re actually looking at something far more special than glowing stars. Every white dot you see is actually an active supermassive black hole. Each of these black holes swallows matter at the center of a galaxy millions of light-years away. In any case, they can be fully detected in this way.
In the study, published in Astronomy & Astrophysics in February (2021), collecting nearly 25,000 points, astronomers created the most detailed map of black holes to date at low radio frequencies. In the lead role of this success, which lasted for many years, is a radio telescope. Astronomer Francesco de Gasperin, from the University of Hamburg, says they have invented new methods in which they can translate radio signals into an image of the sky.
Black holes, when they so “stop” without doing much, do not emit any detectable radiation, making them much more difficult to find. But when a black hole actively accumulates material –a disk of dust and gas around it– intense forces produce radiation at a large number of wavelengths that we can detect throughout space.
What makes the above photo special is that it covers ultra-low radio wavelengths, as detected by the Low Frequency ARray (LOFAR) in Europe. This interferometric network consists of about 20,000 radio antennas scattered across 52 regions across Europe.
LOFAR is the only existing network of radio telescopes capable of deep, High-Resolution Imaging at frequencies below 100 megahertz and offers unique images of the sky. But since it is a telescope on the Earth’s surface, LOFAR has to overcome a problem that telescopes in space are not affected by: the ionosphere. This layer of the atmosphere is particularly a problem for ultra-low frequency radio waves that can be reflected back into space. For example, at frequencies below 5 megahertzin, the ionosphere becomes opaque (impermeable).
The frequencies that can pierce the ionosphere vary according to atmospheric conditions. To overcome this problem, the research team used supercomputers running algorithms to correct ionosphere-induced interference every four seconds. Given the 256 hours of LOFAR staring skyward, that means a lot of Corrections have been made. It is these corrections that provide such a clean image of such a low-frequency Sky.
These new methods will provide new data on all kinds of astronomical objects and phenomena in any region below 50 megahertz, as well as possibly undiscovered objects. According to the researchers, the method will allow the study of more than 1 million low-frequency radio Spectra by providing unique information about galaxies, active nuclei, galaxy clusters and physical models for other areas of research. The team stresses that this experiment is a unique attempt to explore the ultra-low frequency sky at high angular resolution and depth.