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The Cosmic Evolution Survey (COSMOS)

by Dave Sanders

The most detailed 3-D map of the Universe ever made. The COSMOS team used HST and Mauna Kea data to construct the distribution of matter over the past 7 billion years. It shows that the Universe started out with a smooth distribution of matter and became increasingly clumpy as billions of years passed. Credit: Modified from NASA, ESA, R. Massey (California Institute of Technology).

graduate student and postdocs
IfA graduate student Jeyhan Kartaltepe(top) and three postdoctoral fellows, Olivier Ilbert, Emeric LeFloc'h, and Lisa Kewley, are among those working on the COSMOS project.

For the past two years, several students and faculty at the Institute for Astronomy have been working with an international team of about 80 astronomers on a project that promises to revolutionize our understanding of the origin and evolution of galaxies and their relation to the weblike, large-scale distribution of dark matter that astronomers now believe defines the three-dimensional structure of our Universe.

The Cosmic Evolution Survey (COSMOS) had its beginnings in late 2004 as the largest single project ever awarded observing time on the Hubble Space Telescope (HST). COSMOS used nearly 1,000 hours of HST time to survey a two-square-degree region of the sky at optical wavelengths using the Advanced Camera for Surveys instrument, allowing over two million galaxies to be identified in unprecedented detail. The principal investigator for the HST-COSMOS project is Nicholas Scoville (California Institute of Technology), who was a visiting colleague at IfA for nearly a year during 2004-5.

To assemble a true three-dimensional picture of the galaxies, and to understand the sources of energy within individual galaxies, the COSMOS team has been busy competing for observing time to obtain data at wavelengths across the electromagnetic spectrum-- radio, submillimeter, infrared, optical, ultraviolet, and X-ray. Imaging from Mauna Kea has been particularly important: 21 separate filters on the 8-meter Subaru Telescope (Subaru-COSMOS: Yoshi Taniguchi, Japan), ultraviolet observations with the MegaCam imager on the 3.6-meter Canada-France-Hawaii Telescope (CFHT) (U-COSMOS: Dave Sanders, IfA), and near-infrared observations using IfA time on three Mauna Kea telescopes--CFHT, the 3.8-meter United Kingdom Infrared Telescope, and the 2.2-meter UH telescope (N-COSMOS: Dave Sanders, IfA).

Many of the successful projects have been among the largest time allocations ever awarded on their respective telescopes. The projects undertaken with space observatories and their principal investigators include the Spitzer Space Telescope for infrared observations (S-COSMOS: Dave Sanders, IfA), X-ray observations with the Chandra X-Ray Observatory (C-COSMOS: Martin Elvis, Harvard University) and the XMM Space Telescope (X-COSMOS: Gunther Hasinger, Germany), and ultraviolet observations with the GALEX spacecraft (G-COSMOS: David Shiminovich, Columbia University). Projects with ground-based telescopes also include radio observations using the Very Large Array (VLA-COSMOS: Eva Shinnerer, Germany) and optical spectroscopy with the European Southern Observatory's 8-meter Very Large Telescope in Chile (V-COSMOS: Simon Lilly, Switzerland). This unprecedented multiwavelength dataset will eventually be made available to the entire astronomical community for the benefit of all astronomers who wish to study the COSMOS field.

The imaging data from Mauna Kea have been a key to providing one of the most important initial results from the COSMOS project. In a paper recently published in the journal Nature (Richard Massey et al.) and an article that will appear in the COSMOS special issue of The Astrophysical Journal Supplement Series (Scoville et al.), the COSMOS team has been able to derive a consistent picture of both the three-dimensional distribution of galaxies and the distribution of the mysterious dark matter, an invisible form of matter that accounts for most of the Universe's mass and acts as scaffolding inside of which stars and galaxies have been assembled over billions of years. Galaxies and primordial globs of gas and dust are thought to form within the dense clusters and filaments that make up this "cosmic web." For more details:

These two false-color images of the COSMOS field compare the distribution of normal matter (red, left) with dark matter (blue, right) in the Universe. The brightness of clumps corresponds to the density of mass. The mass of the dark matter is six times that of the visible matter. The area of sky surveyed covers nine times that of the full Moon. Credit: Modified from NASA, ESA, R. Massey (California Institute of Technology).

The next few years will be an extremely exciting time for the COSMOS team as the enormous quantity of data being obtained will be assembled and studied to learn about the detailed properties of galaxies as they form and as they are transformed by collisions and mergers with their neighbors and with exotic phenomena--superbursts of star formation, quasars, radio galaxies, and blazers--that make up the current "zoo" of extremely powerful extragalactic objects. Scientists hope that COSMOS will finally be able to construct a coherent picture of all galaxy formation and how galaxies live their lives throughout cosmic time.