Canada-France-Hawaii Telescope Legacy Survey
Reveals Dark Secrets of the Universe

October 26th, 2012

Astronomers from France and Canada have publicly released the final version of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS), a unique and powerful multi-color collection of data obtained over 6 years from the summit of Mauna Kea. The imaging project probes an extremely large volume of the Universe, gathering tens of millions of galaxies, some as far as 9 billion light-years away, and provides a treasure trove for many years of astronomical research. This remarkable collection of data is a landmark achievement for CFHT and has inspired observatories around the world. The large number of published results from these images include dark matter maps on the largest scale ever observed and the first high-quality measurements which showed that dark energy closely resembles the cosmological constant that Albert Einstein predicted, but later thought might have been his greatest mistake.

The Legacy Survey observations began in 2003 and ended in 2009. Three more years were needed to accurately calibrate the huge volume of high-quality and homogeneous data obtained in five color-bands covering the optical domain from blue to red, including near-ultraviolet and near-infrared. The data revealed some 38 million objects, mostly very distant galaxies in various stages of evolution, across a combined area of sky totaling 155 square degrees (800 times the surface area of the full Moon as seen in the sky).

A wallpaper pattern of galaxies
[Full resolution: 1000x939]

This tiny fraction of a CFHTLS Deep field reveals a wallpaper pattern of galaxies. At least a thousand distant galaxies can be identified on this image as little fuzzy dots (the crossed type disks are foreground stars from our own Galaxy). The entire CFHTLS revealed tens of millions galaxies like these.
© CFHT / Coelum


Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/IRFU, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique (CNRS INSU) of France, and the University of Hawaii. This work is based on data products produced at Terapix available at the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.


The Legacy Survey's prime scientific objective was to precisely measure several hundred Type Ia supernovae to help better understand the nature of dark energy. Dark matter and dark energy dominate the Universe, but neither can be seen or identified.

Astronomers, however, can measure dark energy's effect on the rate of cosmic expansion; unlike gravity, the force of dark energy increases as space expands. To measure this influence in the early Universe, astronomers turn to Type Ia supernovae, which are excellent standard candles for measuring galaxy distances.

Between 2003 and 2008, based on the Legacy Survey imaging data, the Supernova Legacy Survey (SNLS) component detected and monitored about 500 Type Ia supernovae spread over the past 8 billion years of the Universe and analyzed the images of more than 10 million galaxies. The results are leading to a better understanding of the early Universe, as well as a determination of the dark energy parameters with an unprecedented accuracy.

Left: A distant supernova.
A typical Type Ia supernova like the many hundreds detected during the supernova survey of millions of distant galaxies. Credit: SNLS
Right: A supernova timelapse.
As shown on the video, the supernova reaches its peak very quickly (a few days) and then slowly fades out over weeks to months. At its peak, a supernova can shine brighter than all the other stars combined in the host galaxy. The animation spans about 4 months, from pre- to post-supernova status. Credit: SNLS

The second major and related core objective was to map the mysterious dark matter on the largest scale ever observed. This required the key combination of deep observations over wide areas of the sky and the development of advanced data-analysis methods.

Galaxies are known to clump together into a giant cosmic web spanning billions of light-years. Known as cosmic shear (the bending of light caused by the dark matter associated with the large scale structures of the Universe), the effect is a subtle form of gravitational-lensing, a phenomenon predicted by Einstein's theory of relativity.

This component of the CFHTLS was the logical next step after a team of astronomers using the CFHT discovered that signature in 2000. The Legacy Survey enabled astronomers joined worldwide in the CFHTLenS Consortium to piece together the geometry of the Universe and learn how or if it will end.

"The Legacy Survey has already generated a lot of results and is the most heavily cited work from CFHT," says Raymond Carlberg, a professor and researcher at the University of Toronto.

The dark matter cosmic web

The ubiquitous dark matter cosmic web as measured by the CFHTLenS Consortium is seen in all four directions surveyed by the Canada-France-Hawaii Telescope. The lower left color inset shows the previous largest COSMOS dark matter map (credit: NASA, ESA, P. Simon and T. Schrabback) and the full Moon to scale. Credit: L. Van Waerbeke (UBC) and C. Heymans (UE)


At the heart of the survey was a state-of-the-art 340-Megapixel digital camera built by the Commissariat à l'Energie Atomique (CEA) of France, called MegaCam, the world's largest at the time. Coupled to the 3.6-meter CFHT atop Hawaii's Mauna Kea, MegaCam captures a one square degree of sky in a single shot with incredible detail.

Left: MegaCam, a 340 Mpx digital camera
The MegaCam focal plane is composed of 40 e2v CCDs. Only the 36 central CCDs making a square footprint are used for science. Credit: J.-J. Bigot (CEA)
Right: The 3.6-meter CFHT with MegaPrime
MegaCam is mounted at the prime focus (called MegaPrime) of the CFHT, located at the top of the telescope (black structure). Credit: J.-C. Cuillandre (CFHT)

From 2003 to 2005, the Legacy Survey also included a very-wide initiative aimed at discovering Solar System bodies beyond the orbit of Neptune, in a region called the Kuiper Belt. The program succeeded in collecting an unprecedented sample of minor bodies in the region before it was continued by the Canada-France Ecliptic Plane Survey (CFEPS). That new initiative used CFHT and other telescopes to determine with high-precision orbits for nearly 200 the Kuiper Belt objects. Astronomers are now using all the data to test models of our Solar System's formation.

In the end, the survey produced four independent MegaCam ultra-deep pointings to a limiting red magnitude of 27 for distant galaxies (about 200 million times fainter than the eye can see without optical aid). It also created a series of mosaics comprised of 171 MegaCam deep pointings which, due to overlaps between adjacent fields, consists of a total of 155 square degrees in the four independent contiguous patches across the northern sky.

As released, the Legacy Survey is composed of more than 15,000 individual MegaCam images, each image represents nearly 1 gigabyte of data. The large amount of data generated at the telescope over the years was processed at Terapix (Institut d'Astrophysique de Paris, UMR 7095, UPMC and CNRS INSU), the curator of the CFHTLS, which characterized and optimized the data set based on individual images calibrated by CFHT. Their main product consists in a series of catalogs of some 38 millions objects.

"Those are some pretty cool numbers considering we are still sampling a small area of the whole sky, but at extreme depths," quips CFHT astronomer Jean-Charles Cuillandre, the MegaCam and Legacy Survey specialist.

A myriad of galaxies - click here to load the full resolution [8601x4838]

Zoom in and then use the horizontal and vertical scrollbars in your browser to navigate the image. This large image covers in whole only half of a single Deep field (0.3% of the entire Legacy Survey sky coverage) and reveals a myriad of galaxies waiting for you to explore. © CFHT/Coelum/Terapix/AstrOmatic

Terapix's mission was to produce refined data products for the scientific communities and the task has now been completed. Every astronomer across the world can gain easy access directly to it from the Canadian Astronomy Data Centre (CADC, NRC). In addition, by feeding the CFHTLS to the Virtual Observatory, searchable catalogues are made available at the Centre de Données Astronomiques de Strasbourg (CDS, UMR 7550, Université de Strasbourg/CNRS) through VizieR.

Now that Legacy Survey's curated data have been made public, the survey can also be explored visually through the CDS Virtual Observatory's Aladin sky atlas. Anyone can explore its deep and shallow secrets. Owing to special software like that used in Google Maps, Aladin users can zoom in and out at their discretion to study huge Legacy images. A comparison in art would be standing close to a Monet and studying individual brushstrokes, then stepping back to take in the full painting.

The CFHTLS fields across the northern sky

The footprint of the four Deep and the four Wide fields from the CFHTLS projected on the entire northern sky observable from Mauna Kea (latitude +20). The spherical projection from the celestial pole appears to squash the Wide fields at lower latitudes. To be able to continuously collect data throughout the years as Earth revolved around the Sun, four fields in four opposite directions in the sky were selected. In two seasonal cases, the Deep field overlaps with the Wide field. Notice how the galactic plane from our galaxy, the Milky Way, was avoided in order to peek into the deep Universe.
Credit: CFHT/Terapix/CDS


Since its inception 34 years ago, CFHT has had a long story of serious impact on cosmology. For instance, between 1992-1994, before 8- and 10-meter-class telescopes appeared on Mauna Kea, CFHT produced the first systematic study of distant, normal galaxies (those like our Milky Way) that gave us the first real glimpse at what the Universe around us would have looked like had we lived when it was only half its present age. Called the Canada-France-Hawaii Redshift Survey, it proved that deep, high-quality surveys can have a great impact on astronomy by pushing the systematic study of the Universe to the faintest possible levels.

CFHT also revealed the first giant gravitational arc - the distorted image of a distant galaxy (one too faint to see otherwise) whose light is magnified, bent, and brought into view by gravitational-lensing effects . CFHT later took the first spectral signature of that giant arc providing the definitive demonstration that it was a "mirage."

Although CFHT commands a prominent perch atop the 4,200-meter-high summit of Mauna Kea - the Northern Hemisphere's premiere observing site for optical, infrared, and submillimeter astronomy - it is now flanked by several 8- and 10-meter-class telescopes.

Left: Mauna Kea
Mauna Kea is a dormant Hawaiian volcano rising 4,200 meters (14,000 feet) above the Pacific Ocean. The clear dark skies and stable atmosphere at the summit provide pristine conditions for astronomy, making it the best ground-based observing site known in the Northern Hemisphere. Credit: J.-C. Cuillandre (CFHT)
Right: The Canada-France-Hawaii Telescope dome
CFHT is a world class 3.6-meter telescope funded according to a tripartite agreement signed in 1974 between Canada (NRC), France (CNRS INSU), and the University of Hawaii. Credit: J.-C. Cuillandre (CFHT)

Given that competition is stiff in astronomy, especially for observing time at a world-class telescope on Mauna Kea, if a small observatory doesn't want to enter the twilight of its life, its astronomers have to be ingenious to find ways to stay in the game. Understanding that a 3.6-meter could not compete in such an arena with the larger telescopes, the CFHT scientific communities took an incredible gamble.

Following on the successful suite of wide-field CCD cameras built at CFHT in the 1990s, they created what was to become the largest astronomical camera for years to come, MegaCam. Canada and France then devoted a significant fraction (35%) of their prime telescope time for the Legacy Survey project, requiring an investment of more than 2500 hours over 6 years (2003-2009), or an equivalent of 450 nights.

The gamble paid off. The Legacy Survey has not only produced the most journal citations in the history of the telescope, but CFHT's scientific impact, as reflected in published research, now ranks among the top 10 most productive telescopes in the world!


"CFHT's legacy has been built on a certain amount of risk taking for decades. MegaCam and the CFHTLS are great examples of how technical innovation and a coherent scientific vision within the CFHT community have led to fantastic successes." states Doug Simons, CFHT executive director.

"We are not yet done with the CFHTLS science!," asserts Hervé Aussel of the French Centre National de la Recherche Scientifique (CNRS) in Saclay. "That's the meaning of the word Legacy. There are new research programs that are starting to use the data from the survey, complementing it with new observations. For example, there is a team looking for the most distant quasar in the Universe in the CFHTLS Wide fields, using the survey data and adding infrared data (CFHQSIR)."

"Without the CFHTLS, we would never be where we are," admits Carlberg. "Tackling the core scientific challenges led the various teams to investigate new methods in astronomical data processing and calibration: this will remain another important legacy that future surveys will most certainly rely on."

Aussel agrees. "The CFHTLS is an extremely successful survey," he says, noting that in 2009, CNRS awarded silver medals to two scientists leading the CFHTLS effort: one to Pierre Astier (IN2P3/CNRS) for the supernova science, and the other to Yannick Mellier (Institut d'Astrophysque de Paris) for the gravitational lensing science.

"The legacy will not be limited to follow-ups of the survey," says Mellier, who leads the Euclid Consortium contributing to the European Space Agency's Euclid mission - a space telescope with cameras designed to accurately measure dark energy and bring its nature to light. He adds, "MegaCam and the CFHTLS truly paved the way for the Euclid space mission both from the scientific and technical aspects."

Scientific impact: the Astier et al. 2006 example

The most referenced scientific work based on CFHTLS data is the dark energy article by Astier and colleagues, which was published in 2006 in the Astronomy & Astrophysics journal. As of October 2012, it racked up more than 1,550 citations (represented on this crowded figure by lines pointing to the articles referencing the SNLS article). When one looks at the "bounce factor" of an article, the average number of citations of all articles referencing this seminal work, one reaches a factor of 33 (more than 50,000 citations total), a clear display of the activity in this field of study. Credit: ADS/PaperScope

Appendice: major scientific impacts

Some significant Legacy Survey research beyond the direct cosmological outcome of the supernovae and lensing surveys is listed below. Please note this list in not exhaustive: hundreds of scientists have worked on the CFHTLS data set and a great variety of results have indeed come out.

  • The survey's Deep- and Wide-field imaging revealed 1200 candidate galaxy clusters with various masses, thus notably increasing the number of known high-redshift cluster candidates, which is an important step towards using cluster counts to measure cosmological parameters.
  • The clustering analysis of 3 million galaxies in the Legacy Survey over four independent fields allowed the most accurate measurement of the mass of the dark matter halos in which galaxies reside. The study revealed a preferred halo mass for star formation to be efficient and that this mass evolves through cosmic time.
  • Thanks to the high precision of MegaCam photometry, the Supernova Legacy Survey has now started to perform measurements on the rate of supernovae at different cosmological epochs. This will furnish a measure of the amount of cosmic star formation arising over large volumes of space and encompassing a large number of galaxies.
  • CFHT's Strong Lensing Legacy Survey has recently compiled a selection of 127 lens candidates in the 155 square degrees of the CFHTLS Wide survey to study the geometric distortion of gravitational-lensing effects. The results will offer a clue as how mass, in particular dark matter, is distributed in the halo.
  • The excellent image quality of the CFHTLS data allowed researchers to identify major galaxy mergers based on the presence of tidal tails and bridges. A study of some 1,600 merging systems revealed that the visible fraction of merging galaxies evolves over time and also that star formation is triggered at all phases of a merger, with larger enhancements at later stages.



  • The support astronomer for MegaCam and CFHTLS is Dr. Jean-Charles Cuillandre.
    You can contact him about this story at