Hubble Helps Map Dark Matter In 3D
October 17, 2012

Researchers Study Dark Matter Filament In 3D

April Flowers for - Your Universe Online

A giant filament of dark matter extends 60 million light-years from one of the most massive galaxy clusters known. Leftover from the very first moments after the Big Bang, the filament is part of the cosmic web that constitutes the structure of the Universe. Scientists using the NASA/ESA Hubble Space Telescope have studied this filament in 3D for the first time and conclude that if the filament is representative of the rest of the cosmic web, these structures might contain more than half the mass of the entire Universe.

According to the Big Bang theory, variations in the density of matter from those first moments condensed the bulk of the matter into a web of tangled filaments, which connect at the locations of massive galaxies. Computer simulations have supported this theory, but the filaments are mostly made of dark matter, which is notoriously hard to observe.

Earlier this year, scientists made the first convincing identification of a section of one filament. The current team of astronomers has pushed this further by probing the filaments structure in 3D, which eliminates many of the problems associated with studying a flat image of such a structure.

“Filaments of the cosmic web are hugely extended and very diffuse, which makes them extremely difficult to detect, let alone study in 3D,” says Mathilde Jauzac of LAM, France and University of KwaZulu-Natal, South Africa.

High-resolution images of the massive galaxy cluster MACS J0717.5+3745 (or MACS J0717 for short) obtained from the Hubble, NAOJ's Subaru Telescope and the Canada-France-Hawaii Telescope were combined with spectroscopic data on the galaxies inside the cluster provided by the WM Keck Observatory and the Gemini Observatory to give the team a complete view of the shape of the filament.

The team likens their methodology to a recipe with four crucial ingredients.

First, add in a promising target. Filaments slowly funnel matter into the clusters where the filaments of the cosmic web meet, according to current theories.

“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 Harald Ebeling of the University of Hawaii at Manoa, who led the team that discovered MACS J0717 almost a decade ago.

Second, slowly fold in advanced gravitational lensing techniques. Einstein's theory of general relativity states that the path of light is bent when it passes through or near objects with a large mass. Dark matter, which comprises the bulk of the filaments, cannot be seen directly. However, their mass is enough to bend the light and distort images of galaxies in the background. This process is called gravitational lensing, and the team has developed new tools to convert the image distortions into a mass map.

Next, combine the earlier ingredients with high resolution images. Studying gravitation lensing requires very detailed images because it is a subtle phenomenon. The team was able to study the precise deformation in the shapes of numerous lensed galaxies using Hubble observations, which in turn revealed where the hidden dark matter filament is located.

“The challenge,” explains Jean-Paul Kneib of LAM, France, “was to find a model of the cluster´s shape which fitted all the lensing features that we observed.”

For the final ingredient, measurements of distances and motions are necessary. The original Hubble observations give the best 2D map of a filament, but to truly see its shape in 3D, the astronomers needed additional observations. Galaxy velocity measured with spectrometers from the Subaru, CFHT, WM Keck, and Gemini North telescopes along with color images allowed the team to locate thousands of galaxies within the filament. It also allowed the scientists to detect the motions of many of those galaxies.

Combining velocity and positional information for all these galaxies, the scientists created a model that then revealed the 3D shape and orientation of the structure of the filament. This allowed the team to work without the uncertainties and biases of working with a 2D image, enabling them to measure the true properties of the elusive filamentary structure.

This study pushes the limits of predictions made by previous theoretical work and numerical simulations of the cosmic web. The MACS J0717 filament is extreme even on astronomical scales with a length of at least 60 million light-years. The results suggest that earlier predictions of the mass contained in such filaments are off by a great deal.

Astronomers are awaiting the launch of the NASA/ESA/CSA James Webb Space Telescope in 2018. It will be a powerful tool for detecting filaments in the cosmic web because of its greatly increased sensitivity.

The findings of this study were published in Monthly Notices of the Royal Astronomical Society.

Image 2 (below): This diagram shows Hubble´s image of MACS J0717 overlaid with the location of the cluster and filament´s mass (in blue), and with a schematic of the cluster and filament (in white). Below, the diagram shows the geometry from our perspective in which the filament lies almost directly along our line of sight, and from a different perspective in which the filament is seen side-on. This shows how the filament appears greatly foreshortened from our perspective. Credit: NASA, ESA, Harald Ebeling (University of Hawaii), Karen Teramura (University of Hawaii)