Galaxies and dark matter in the filament around J0717: This enormous image shows Hubble’s view of massive galaxy cluster MACS J0717.5+3745. The large field of view is a combination of 18 separate Hubble images. Studying the distorting effects of gravity on light from background galaxies, a team of astronomers has uncovered the presence of a filament of dark matter extending from the core of the cluster. The location of the dark matter is revealed in a map of the mass in the cluster and surrounding region, shown here in blue. The filament visibly extends out and to the left of the cluster core. Using additional observations from ground-based telescopes, the team was able to map the filament’s structure in three dimensions, the first time this has ever been done. The filament was discovered to extend back from the cluster core, meaning we are looking along it.

Image credit: NASA, ESA, Harald Ebeling & Jean-Paul Kneib

An exciting new study lead by Dr Mathilde Jauzac, an astronomer at the University of KwaZulu-Natal, and an international team of astronomers has for the first time unveiled the three-dimensional structure of a cosmic filament using high-resolution images from the Hubble Space Telescope. The cosmic filament forms part of the cosmic web that traces the distribution of matter in the Universe,similar to the complex system of neurons in our brain; in this picture filaments play the role of nerve fibres.

The international team comprising scientists from South Africa, France, USA, Japan, Denmark and theUnited Kingdom, discovered the filament when studying the distribution of mass around the massive galaxy cluster MACS J0717.5+3745 (or MACS J0717 for short). Galaxy clusters are the largest structures in the Universe held together by gravity, contain thousands of galaxies and are believed to lie at the nodes of the cosmic web. In time they grow as matter is funnelled into them along cosmic filaments. “From our earlier work on MACS J0717, we knew that this cluster is actively growing, and thus a prime target for a detailed study of the cosmic web,” explains co-author Harald Ebeling from the University of Hawaii in the USA.

Dr Jauzac, who earlier this year started a research fellowship at the Astrophysics and Cosmology Research Unit at the University of KwaZulu-Natal, used the method of gravitational lensing to map out the distribution of the dark matter in the cosmic filament. Dark matter, which has eluded detection by astronomers to date, makes up ninety percent of the matter in the Universe and forms the backbone of the cosmic web. The technique of gravitational lensing has its foundation in Einstein’s theory of general relativity, which predicts that light should bend around large concentrations of mass; in this way the galaxy cluster acts as a large gravitational lens. It is a subtle effect and the study required very precise images from the Hubble Space Telescope of the galaxies around and behind the filament, and new tools developed by the team, to reveal the hidden dark matter filament. “The challenge,” explains co-author Jean-Paul Kneib from the Laboratoire d’Astrophysique de Marseille in France “was to find a model of the cluster’s shape which fitted all the lensing features that we observed.”

Observations by the Hubble Space Telescope gave the best two-dimensional map of the mass distribution but to see the shape of the filament in three dimensions required further observations from the Subaru and Canada-France-Hawaii telescopes to locate thousands of galaxies within the filament, and measurements of their velocities using the Keck and Gemini telescopes. Dr Jauzac, who recently obtained her PhD from Laboratoire d’Astrophysique de Marseille in France, pointed out that “Filaments of the cosmic web are extremely extended and diffuse, which makes them very difficult to detect, let alone study in three dimensions”. The first convincing detection of part of a cosmic filament was made earlier this year by a team of astronomers led by Jorg Dietrich. The filament discovered by Dr Jauzac’s team is so extended that a beam of light would take about 60 million years to travel across it.

The team’s findings have backed up the view of how small irregularities in the Universe at the time of the big bang grew over billions of years to form the large cosmic structures that we see today, and have provided astronomers with further insights into how the elusive dark matter is spread across the Universe. Professor Sunil Maharaj, director of the Astrophysics and Cosmology Research Unit confirmed the impact of this study by saying that, “The team have shown insight and originality in making an excellent contribution to a difficult problem. We are indeed pleased that Dr Jauzac from our research unit at the University of KwaZulu-Natal has led this pioneering study”.

Dr Jauzac plans to continue her work on a larger sample of galaxy clusters, with similar interesting features, using observations from the Southern African Large Telescope (SALT). The forthcoming James Webb Space Telescope will be a powerful tool for detecting filaments in the cosmic web, thanks to its enhanced sensitivity over the Hubble Space Telescope.

The Hubble Space Telescope is a project of international cooperation between ESA and NASA. 

The international team of astronomers in this study consists of Mathilde Jauzac (University of KwaZulu-Natal, South Africa, and Laboratoire d’Astrophysique de Marseille, France), Eric Jullo (Laboratoire d’Astrophysique de Marseille, France and Jet Propulsion Laboratory, USA), Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille), Harald Ebeling (University of Hawaii, USA), Alexie Leauthaud (University of Tokyo, Japan), Cheng-Jiun Ma (University of Hawaii), Marceau Limousin (Laboratoire d’Astrophysique de Marseille and University of Copenhagen, Denmark), Richard Massey (Durham University, UK) and Johan Richard (Lyon Observatory, France).

The research is presented in a paper entitled “A Weak-Lensing Mass Reconstruction of the Large-Scale Filament Feeding the Massive Galaxy Cluster MACSJ0717.5+3745”, to be published in the 1 November 2012 issue of Monthly Notices of the Royal Astronomical Society.

The Astrophysics and Cosmology Research Unit (ACRU), based at the University of KwaZulu-Natal, aims to promote and maintain a positive and thriving research unit through its academic staff and pioneering research programme. ACRU members include senior researchers, postdoctoral fellows, postgraduate students and undergraduate students.