A wide US – Europe network of radio telescopes including observing a faraway radio galaxy discovers a small dark object. It is just a pinch in their data but it signifies that low-mass dark matter clumps are widespread, verifying theories of the Universe’s structure and evolution. The telescopes used include those of VUAS institute “Ventspils International Radio Astronomy Centre” (VIRAC).

An international team of astronomers has found a low mass dark object in the distant Universe, not by directly observing any emitted light, but by detecting its tiny gravitational distortion of the light from another distant galaxy. This mysterious object has a mass of about one million times that of our Sun, and its discovery seems consistent with the current best theory about how galaxies like our own Milky Way formed.

“Since we can’t see dark matter objects directly, we instead use very distant galaxies as a backlight to look for the gravitational imprints of such objects.”

“Hunting for dark objects that don't seem to emit any light is clearly challenging,” said Dr. Devon Powell at the Max Planck Institute for Astrophysics (MPA) and lead author of the study published in Nature Astronomy. “Since we can’t see them directly, we instead use very distant galaxies as a backlight to look for their gravitational imprints.”

Dark matter is an enigmatic form of matter not expected to emit light, yet it is essential to understanding how the rich tapestry of stars and galaxies we see in the night sky evolved. As a fundamental building block of the universe, a key question for astronomers is whether dark matter is smooth or clumpy, as this could reveal what it is made of. As dark matter cannot be seen, its properties can only be determined by observing the gravitational lensing effect, whereby the light from a more distant object is distorted and deflected by the gravity of the dark object, creating a characteristic arc in the processed image.

The team used a network of telescopes from around the world, including the Green Bank Telescope (GBT), the Very Long Baseline Array (VLBA) and the European Very Long Baseline Interferometric Network (EVN). The latter includes the VIRAC radio astronomical observatory in Irbene. The data from this international network were correlated at the Joint Institute for VLBI ERIC (JIVE) in the Netherlands, forming an Earth-sized super-telescope that could capture the subtle signals of gravitational lensing by the dark object. They discovered that the object has a mass a million times greater than that of our Sun and is located in a distant region of space, approximately 10 billion light years from Earth, when the universe was only 6.5 billion years old.

“From the first high-resolution image, we immediately saw a pinch in the gravitational arc, which is the tell-tale sign that we were onto something.”

This is the lowest mass object to be found using this technique, by a factor of about 100. To achieve this level of sensitivity, the team had to create a high-fidelity image of the sky using radio telescopes located around the world. Professor John McKean from the University of Groningen (RuG), the University of Pretoria (UP) and the South African Radio Astronomy Observatory (SARAO), who led the data collection and is the lead author of a companion paper, said: “From the first high-resolution image, we immediately saw a pinch in the gravitational arc, which is the tell-tale sign that we were onto something. Only another small clump of mass between us and the distant radio galaxy could cause this.”

To analyse the massive dataset, the team had to develop new modelling algorithms that could only be run on supercomputers. “The data are so large and complex that we had to develop new numerical approaches to model them. This was not straightforward as it had never been done before,” said Dr Simona Vegetti at MPA. “We expect every galaxy, including our own Milky Way, to be filled with dark matter clumps, but finding them and convincing the community that they exist requires a great deal of number crunching,” she continued. The team applied a special technique called gravitational imaging, which allowed them to ‘see’ the invisible dark matter clump by mapping its gravitational lensing effect against the radio-luminous arc.

“We expect every galaxy, including our own Milky Way, to be filled with dark matter clumps”

“Given the sensitivity of our data, we were expecting to find at least one dark object, so our discovery is consistent with the so-called ‘cold dark matter theory’ on which much of our understanding of how galaxies form is based,” said Powell. “Having found one, the question now is whether we can find more and whether the numbers will still agree with the models.”

The team are now analysing the data further to better understand what the mysterious dark object could be, but they are also looking into other parts of the sky to see if they can find more examples of such low-mass dark objects using the same technique. If they continue to find such mysterious objects in other parts of the universe, and if they really turn out to be completely devoid of stars, then some theories of dark matter may be ruled out.