Super-telescope bring astronomers closer to understanding dark matter

Astronomers using a global network of radio telescopes have produced one of the sharpest astronomical images ever. The resulting image demonstrates that dark matter is distributed unevenly across a distant galaxy. Their findings are recently published in the Monthly Notices of the Royal Astronomical Society.

The image was created by combining data from a global radio telescope network, comprised of the European VLBI Network, and the Very Long Baseline Array and Green Bank Telescope in the United States, in an effort to address some of the fundamental questions about dark matter. The international team of astronomers aim to determine how much dark matter is present in galaxies and how it is distributed. According to current theories, a galaxy, such as our Milky Way, should have thousands of dwarf galaxies orbiting around it, yet to date only approximately 100 have been found.

"It has been suggested that these dwarf galaxies could be dark matter dominated and, therefore, highly difficult to observe. However, throughout the distant Universe, we can discover the presence of these small mass structures only by using the gravitational lensing effect," explains Cristiana Spingola, lead author on the paper, from the Kapteyn Astronomical Institute, Groningen.

Gravitational lensing allows astronomers to observe incredibly distant radio sources that cannot be directly detected. By observing how the radio emission from the distant source is bent by the gravitational field of a massive object - the lens - located between the source and the Earth, it is possible to determine information about both the distant source and the lens. In this study the researchers used the radio source MG J0751+2716, at such great a distance that it has taken the light 11.7 billion years to reach the Earth. This object is comprised of a black hole with a powerful ejection of material, known as a jet. The lens consists of a group of galaxies located at a look-back time of 3.9 billion years from Earth.

In the study, the astronomers were able to determine the distance, brightness and projected size of the radio source, together with the composition of dark matter across the lens, which appeared clumpy and uneven.

"For the first time we were able to observe large gravitationally lensed arcs on extremely small angular scales. The background source - the black hole with radio jets - is distorted into these arcs on the image because of the gravitational effect of the foreground galaxies (the lens). It is a rare possibility to get such an extended arc." Spingola added, "the unprecedented detail of these extended gravitational arcs allowed us to infer with high precision the distribution of the matter of the galaxy acting as a lens."

It is only possible to obtain such high-resolution data by coordinating multiple telescopes to observe the same radio source simultaneously. In this case, 24 radio antennas from across the globe were connected using a technique called Very Long Baseline Interferometry (VLBI). Data from all the telescopes was collated in a process known as correlation, at a super computer housed at the Joint Institute for VLBI ERIC in Dwingeloo, the Netherlands.

To better understand the properties for dark matter, the team are now applying sophisticated numerical algorithms to quantify the nature of the clumpy dark matter. But, they are also on the hunt for more extended gravitational arcs just like this.

"There are only a limited number of gravitational lenses suitable for this study, and while we have started this search using the European VLBI Network and the Very Long Baseline Array we expect that there will be more giant radio arcs in the future," explained John McKean, project lead from the Netherlands Institute for Radio Astronomy (ASTRON) and the Kapteyn Astronomical Institute.

This study was led by John McKean from the Kapteyn Astronomical Institute, University of Groningen and the Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo on behalf of the international research team SHARP (Strong Lensing at High Angular Resolution Project) led by Chris Fassnacht (University of California, Davis), and also including Matt Auger (University of Cambridge), Leon Koopmans (University of Groningen), David Lagattuta (University of Lyon) and Simona Vegetti (Max Planck Institute for Astrophysics).

Reference: C. Spingola, J. P. McKean, M. W. Auger, C. D. Fassnacht, L. V. E. Koopmans, D. J. Lagattuta, S. Vegetti: SHARP – V. Modelling gravitationally-lensed radio arcs imaged with global VLBI observations DOI: 10.1093/mnras/sty1326