CFHT Gives First Glimpse of Dark Matter Distribution


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  Table of contents

  Press release

Kamuela, Hawaii, March 7, 2000

An international team of astronomers based in France has obtained the first-ever glimpse of the distribution of dark matter over a large section of sky. The team used images from the Canada-France-Hawaii Telescope's high-resolution wide-field imaging camera to analyze the light of 200,000 distant galaxies, looking for distortions caused by intervening dark matter. The results give cosmologists their first clear window into the possible roles of dark matter in the evolution of the Universe.

The 13-member team, headed by Dr. Yannick Mellier of the Institut d'Astrophysique de Paris and the Observatoire de Paris, drew on a wide range of expertise, including cosmology, astrophysics, statistics, data analysis and instrument technology by bringing together researchers from France, Germany, Canada and the United States.
The nature of dark matter is one of the greatest unsolved mysteries of modern science. While dark matter makes up at least 90% of the mass of the Universe, both its composition and its distribution are unknown. Knowledge of dark matter is, however, critical to understanding the evolution and fate of the Universe.
"In cosmology we develop models to try and understand what processes underlie the evolution of the Universe," explains Dr. Ludovic van Waerbeke of the Canadian Institute for Theoretical Astrophysics in Toronto. "We want to know, for example, how galaxies evolved, why we see great voids in space, what is causing galaxies to cluster in large filaments and sheets."
Cosmologists also want models that predict the fate of the Universe. At issue is whether the Universe will expand forever, contract and collapse, or oscillate between expansion and contraction. But without a knowledge of dark matter, the major constituent of the Universe, accurate models are difficult to build.
"To build cosmological models we need to have an idea of the total matter content of the Universe," says Dr. Yannick Mellier, the team's leader. "Since somewhere around 90% of this matter is invisible, it's hard for us to get a precise reading on this. Also, to test our models, to see if they accurately describe the Universe we need to look at the results of our simulations against what is actually out there, what astronomers really see."
But, says Mellier, up until now astronomers could see the distribution of only 10% of the matter in the Universe, making it difficult to judge the accuracy of different models.
To determine the distribution of dark matter, Mellier's team used CFHT's wide-field imaging camera CFH12K, one of the largest in the world, to obtain high-resolution images of a two-square-degree section of sky (10 times the surface of the full moon). Using state-of-the-art image analysis software largely developed at the TERAPIX data analysis and processing center in Paris, the team was able to analyze the light from 200,000 very distant and faint galaxies, looking for the minute distortions that, in theory, should occur as the light passes through the gravitational fields of intervening dark matter.
Using this information the team has developed the first ``map'' of dark matter in that area of sky, allowing researchers to visualize how it condensed out of the early universe and distributed itself over the course of time. The analysis has revealed the presence of a vast matrix of interconnected dark matter. The result is not only a significant technological feat, but also a major advance in astronomy and cosmology.
According to Dr. Greg Fahlman, Director of the Canada-France-Hawaii Telescope, the results are but a preliminary view of what the future is promising: "By 2002 we will have a new wide-field imaging camera on the telescope that will cover, with improved sensitivity, an area of sky 3 times greater than the current camera. This new instrument will greatly enhance our ability to map the cosmic distribution of dark matter."
MegaCam, as the camera is called, will provide astronomers with the data they need to develop significantly more accurate models of the universe. "Our goal," Dr. Fahlman adds, "is to help create the first distribution maps of dark matter across the sky, similar to the distribution maps you currently see for galaxies."

  Further scientific description

Observational discovery of cosmic astigmatism caused by Dark Matter in the Universe

For many years, astronomers have believed that 90% of the matter in the Universe is in some mysterious dark form that emits no light of any kind. The cosmic distribution of this dark matter is difficult to study since astronomical equipment can only detect luminous objects, primarily distant galaxies, that constitute a small fraction of the total mass in the large volume of the visible Universe.
An international team based in France has announced the first direct detection of the dark matter by measuring the cosmic astigmatism caused by the gravitational lensing effect first noted by Albert Einstein some ninety years ago. Light rays from distant galaxies are slightly deflected by gravity as they pass through and near clumps of dark matter on their way to the Earth. Consequently, the appearance of a distant galaxy is slightly distorted.
For the first time, this type of distortion has been detected. Using a series of deep images obtained at the Canada-France-Hawaii Telescope over the past two years (Figure 1), the French team analyzed the shapes of some 200,000 faint galaxies spread over two square degrees of the sky (an area approximately 10 times greater than that of the full moon). They have determined that the galaxies appear to be elongated in a coherent manner over large regions of the sky. The measured effect is small, a percent or so deviation from a purely random distribution of shapes, but the accuracy of the results leaves no doubt that the signal is due to the gravitational lensing effect of the dark matter distribution. These results have been partially confirmed by subsequent reports from two teams, one English and the other American, who have studied different patches of the sky.
The measurement of cosmic astigmatism has been the object of a lively international competition. The long term goal is to obtain a detailed map of the large-scale distribution of dark matter. With such a map, our understanding of the evolution of structure in the Universe, which arises from the clumping due to the attractive force of gravity, can be greatly refined. Gravity causes matter to aggregate into long, intersecting filaments surrounded by vast, nearly empty voids (Figures 2 & 3). The precise characterization of these structures by analyzing cosmic astigmatism will reveal the initial conditions that prevailed at the origin of the Universe.
Cosmic astigmatism has been studied extensively over the past decade. The predicted signal is so weak that the prospects of a successful detection seemed very poor but the importance of the measurement was too great to simply abandon the effort. In order to mount a comprehensive attack on the problem, a research team assembled by the IAP has put in place a program to fully exploit the new wide-field CCD camera at CFHT. The data analysis was performed at the TERAPIX center for high-capacity data analysis at the IAP, where the statistical methodology has been under development for the past five years.
This first observation of the gravitational distortion produced by dark matter is a significant achievement. The result immediately provides some constraints on the amplitude of the dark matter density fluctuations in the early universe but, perhaps more importantly, it demonstrates the feasibility of mapping the dark matter distribution across large areas of the sky. A new instrument for the CFHT known as MegaCam is under construction by the astrophysics group at CEA, HIA in Canada and the CFHT. This camera has a field of view three times larger than the present camera and will be able to map vast regions of the sky. Cosmologists will soon have a tool, unique in world, with which to map the dark matter in the Universe and to understand its evolution. A new window for studying the Universe has been opened.

  Images and captions

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``[Low res.]'' images are small size (800x800 pixels) high quality JPEG format images while ``[High res.]'' images are large size (2,500x2000 pixels) high quality JPEG format images.
Figure 1 -    Local [Low res.] - Browser [Low res.] / [High res.]
Probing the Universe - Wide-field imaging at CFHT with the CFH12K camera
Top left:
The whole image shows the entire field of view of the CFHT wide-field camera. The blue circle shows the size of the full moon as it would be imaged. The boxes indicate the relative size of the enlarged areas shown on this figure. Eight of these large patches of the sky (several hours exposure each) were used to conduct this work.
This large area of the sky shows a group of well defined nearby galaxies in front of a background of distant galaxies (the small ellipses of various colors).

Bottom left:
This small fraction of the whole field of view shows in detail the distant galaxies. This area contains more than 100 of them; the whole sample used for this work contains 200,000 galaxies similar to those.

Figure 2 -    Local [Low res.] - Browser [Low res.] / [High res.]
Deflection of light rays crossing the universe, emitted by distant galaxies
Numerical simulation showing the distribution of dark matter in a large volume of the universe. The box shown spans a distance of about 1 billion light-years. The structures are displayed so that the brighter regions have a higher density (that is, more dark matter) than the darker regions. The dark matter is concentrated into a web-like distribution of filaments that intersect at dense nodes where great clusters of galaxies are expected to form and become visible. At the rear of the cube (to the left), three blue disks represent three distant galaxies. The yellow lines that cross the box represent light rays from those galaxies propagating through the universe. In the absence of intervening matter, the light would travel on straight lines but in the presence of matter, the paths of the rays are evidently deflected by the gravitational effects of the clumpy matter (the breaks in the yellow lines illustrate the light passing behind a clump of dark matter). The light from a distant galaxy rarely encounters a clump of mass to strongly bend the light and cause an easily seen distortion. Instead the individual light rays suffer a series of small deflections such that an observer located at the front of the box (to the right), sees that the images of all the galaxies in some small patch of the sky, near to one of our test galaxies say, are all very slightly elongated in a common direction determined by the distribution of dark matter along that particular line of sight. This gravitational distortion is expected to be very small and requires a careful statistical treatment on many patches over the sky but has now been measured by the French team.

This numerical simulation was kindly made available by S. Colombi of the IAP.

Figure 3 -    Local [Low res.] - Browser [Low res.] / [High res.]
Image of the distant galaxies lensed by the dark matter of the universe
This view shows what the observer at the front of the box would percieve when looking at galaxies in the sky. The blue elongated disks are the images of distant galaxies formed by their light after it has passed through the box. The observer can see these galaxies but the filaments of dark matter, shown here in red and white, are invisible, even to the largest telescopes available to our observer. However, one can see that the galaxy images are elongated in a special way on average: they are stretched along a direction parallel to the filaments of dark matter. This effect is a consequence of gravitational lensing which stretches the tight bundle of light rays from a single galaxy much like the moon's gravity stretches the Earth to cause the ocean tides. By measuring the systematic distortion in the images of distant galaxies, one can "see" the dark matter. The ultimate goal of the French team is to map the dark matter with the new CFHT instrument MegaCam, as one of the special survey programs now being planned.

This numerical simulation was kindly made available by S. Colombi of the IAP.

Figure 4 -    Local [Low res.] - Browser [Low res.] / [High res.]
Mapping the dark matter of the universe with gravitational lensing
The numerical simulation on the left is a similar view as the one seen on Figure 3 to the difference that the structures are displayed with the darker regions corresponding to higher density (i.e. more dark matter). The right panel represents an overlay of the left panel and the map of the gravitational lensing caused by these large dark matter structures. The length of each red line indicate the ``force'' of the gravitational lensing: the longer the stronger. The orientation of each of these lines indicates the direction of the gravitational lensing: it is perpendicular to the filaments of dark matter, the galaxy images getting stretched along a direction perpendicular to these lines.
By building the gravitational lensing map through observations of hundreds of thousands of galaxies over large patches of the sky (this figure covers a surface of 5 by 5 square degrees), the astronomers are then able to directly map the dark matter of the Universe.

Contact about images:
   Dr. Jean-Charles Cuillandre
   Canada-France-Hawaii Telescope Corporation
   Phone number: 808 885 7944


  In depth technical description

The following link will take you to the electronic archive and distribution server for research papers (``astro-ph'') where the technical article has been placed.
Detection of correlated galaxy ellipticities on CFHT data:
first evidence for gravitational lensing by large-scale structures

L. Van Waerbeke (CITA), Y. Mellier (IAP, Obs Paris), T. Erben (MPA), J.-C. Cuillandre (CFHT), F. Bernardeau (CEA Saclay), R. Maoli (IAP), E. Bertin (IAP, Obs Paris), H.J. Mc Cracken (LAS), O. Le Fevre (LAS), B. Fort (IAP), M. Dantel-Fort (Obs Paris), B. Jain (JHU), P. Schneider (MPA)
18 pages, submitted to ``Astronomy and Astrophysics'' on the 27th Feb. 2000. Accepted for publication 11th April 2000.

  Scientific Team

The team responsible for this work consists of:
Ludovic Van Waerbeke (1), Yannick Mellier (2,3), Thomas Erben (4), Jean-Charles Cuillandre (5), Francis Bernardeau (6), Roberto Maoli (2), Emmanuel Bertin (2,3), Henry J. Mc Cracken (7), Olivier Le Fevre (7), Bernard Fort (2), Mireille Dantel-Fort (3), Bhuvnesh Jain (8), Peter Schneider (4)

1: Canadian Institute for Theoretical Astrophysics, Toronto, Canada
2: Institut d'Astrophysique de Paris, Paris, France
3: Observatoire de Paris, DEMIRM, Paris, France
4: Max Planck Institut fur Astrophysik, Garching, Germany
5: Canada-France-Hawaii Telescope Corporation, Kamuela, USA
6: Service de Physique Theorique. C.E.A. de Saclay, France
7: Laboratoire d'Astronomie Spatiale, Marseille, France
8: Dept. of Physics, Johns Hopkins University, Baltimore, USA


  Contact Information

Dr. Yannick Mellier
Institut d'Astrophysique de Paris
Phone number: (33) 1 44 32 81 40
      (7AM to 9 AM & 8 PM to 12 PM, LT)
Cellular phone: (33) 6 13 50 77 82
      (6 PM to 9 AM, LT)

Dr. Greg Fahlman
Canada-France-Hawaii Telescope
Phone number: 808 885 7944
      (9 AM to 5 PM, LT)


Contact about this web page and images:
   Dr. Jean-Charles Cuillandre
   Canada-France-Hawaii Telescope Corporation
   Phone number: 808 885 7944


  The Canada-France-Hawaii Telescope

The Canada-France-Hawaii Telescope is funded through the National Research Council of Canada (NRC), the Centre National de la Reserche Scientifique (CNRS) in France, and the University of Hawaii.

  The DESCART project

The Descart project is supported par the Institut National des Sciences de l'Univers of the CNRS and the Programme National de Cosmologie. It has also received support from the Region Ile-de-France through the Centre de Traitement d'Information et de Simulation based at IAP.

  Another form of gravitational lensing

This recent document featured in ``CFHT's Image of the Week'' introduces to a stronger form of gravitational lensing discovered in the mid 80's: lensing caused by massive clusters of galaxies.

  Wide-field imaging at CFHT

Visit the CFH12K CCD mosaic camera web page to find out more about CFHT's most recent instrument.