Mysterious Dark Object In Space

Scientists detect the lowest mass dark object currently measured

To the point

• Gravitational lenses: Distortions caused by gravitational lenses can be used to study the properties of dark matter, even though it does not emit light.
• Discovery: An international team has discovered a dark object in the distant universe that has one million times the mass of the Sun. The discovery is based on an analysis of the gravitational effects on the light from another galaxy.
• Technology: A network of radio telescopes around the world, including the Green Bank Telescope, collected the data. It forms a virtual supertelescope that enables enhanced image quality, allowing even small gravitational signals to be detected.

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. Since dark matter cannot be observed directly, 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. "Hunting for dark objects that do not seem to emit any light is clearly challenging," said Devon Powell at the Max Planck Institute for Astrophysics and lead author of the study. "Since we can't see them directly, we instead use very distant galaxies as a backlight to look for their gravitational imprints."

The team used a network of telescopes from around the world, including the Green Bank Telescope, the Very Long Baseline Array and the European Very Long Baseline Interferometric Network. The data from this international network were correlated at the Joint Institute for VLBI ERIC in the Netherlands, forming an Earth-sized super-telescope that could capture the subtle signals of gravitational lensing by the dark object. They found that the object has a mass that is 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.

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. John McKean from the University of Groningen, the University of Pretoria, and the South African Radio Astronomy Observatory, who led the data collection and is the lead author of a companion paper, stated: "From the first high-resolution image, we immediately observed a narrowing 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."

New modelling algorithms

To analyze 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 Simona Vegetti at the Max Planck Institute for Astrophysics. "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.

"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 their number will still agree with the models."

The team is now analyzing 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.

Additional Information:

Gravitational lensing: This is an astrophysical tool used by astronomers to measure the mass properties of structures in the Universe. It is a consequence of Einstein's Theory of General Relativity, where mass in the Universe curves space. If the mass of the foreground lensing object (typically a galaxy or cluster of galaxies) is sufficiently dense, then the light from distant objects is distorted and multiple images are even observed. In the case of this system, called B1938+666, the foreground infrared-luminous galaxy (seen at the centre of the ring), results in a beautiful Einstein ring of the distant galaxy. However, the distant galaxy is also bright at radio wavelengths, showing the beautiful multiple images and gravitational arcs (seen in red).

Very Long Baseline Interferometry: The radio observations were taken using a combination of radio telescopes that are combined to form a so-called Very Long Baseline Interferometer. This observational method allows astronomers to improve the imaging sharpness of the data and reveal very small fluctuations in the brightness that otherwise could not be seen. For example, the resolving power of the data is a factor of 13 times better than the infrared imaging from the W. M. Keck Telescope adaptive optics system (also shown in the figures in black and white). The telescopes used in the observations were the Green Bank Telescope and the Very Long Baseline Array of the National Radio Astronomy Observatory in the United States, and the telescopes of the European Very Long Baseline Interferometric Network.

Gravitational imaging: This is a novel method that astronomers use to 'see' mass in the Universe even though it does not emit any light. This method uses the extended gravitational arcs to look for small aberrations that can only be caused by an additional, invisible component of mass. By combining this method and the exquisite high angular resolution imaging from the data, the team was able to detect the presence of the lowest mass dark object currently measured.