Three dimensional map of the Universe now available to everyone

August 08, 2012

The Sloan Digital Sky Survey III (SDSS-III) has released the largest-ever three-dimensional map of massive galaxies and distant black holes, which will help astronomers explain the mysterious "dark matter" and "dark energy" that scientists know makes up 96 per cent of the universe. Early last year, the SDSS-III released the largest-ever image of the sky; the new data have begun to expand this image into a full three-dimensional map. "Data Release 9" (DR9), released last week, publically releases the data from the first two years of this six-year project.

Earlier this year, a part of SDSS-III, the BOSS survey, was used by scientists at the Max Planck Institute for Extraterrestrial Physics and other institutes worldwide to map the three-dimensional positions of more than a quarter million galaxies, yielding the most accurate and complete map of the galaxy distribution up to a distance of about 6 billion light years. Using this cosmic web, the researchers were able to reconstruct the expansion history of the universe and to constrain the parameters of the standard cosmological model to an accuracy of better than five per cent. (More information can be found here.)


A still image from a video fly-through of the SDSS-III galaxies mapped in Data Release 9. (You can find the video on the SDSS press release site - for a link see right column.) Galaxies are concentrated into clusters and filaments with voids in between. The SDSS-III is exploring this structure to determine the nature of dark energy and the distribution of dark matter in the Universe.

This map of the universe is also the centrepiece of DR9. The release includes images of 200 million galaxies and spectra of 1.35 million galaxies, including new spectra of 540,000 galaxies from when the universe was half its present age. Spectra show how much light a galaxy gives off at different wavelengths. Because this light is shifted to longer, redder wavelengths as the Universe expands, spectra allow scientists to figure out how much the Universe has expanded since the light left each galaxy. The galaxy images plus these measurements of the expansion are combined by SDSS-III scientists to create the three-dimensional map released with DR9.


Further interesting results can be expected from the analysis of distant “quasars”. These are the brightest objects in the distant universe and their spectra show intricate patterns imprinted by the large-scale clumping of intergalactic gas and underlying dark matter that lies between each quasar and the Earth. Also our own cosmic backyard, the Milky Way galaxy can be studied with DR9 data, which includes better estimates for the temperatures and chemical compositions of more than half a million stars in our galaxy.


The optical data provided by SDSS will also be used to complement an X-ray mission currently under construction at the Max Planck Institute for Extraterrestrial Physics. The eROSITA satellite (to be launched in 2014) will scan the whole sky in X-rays and is expected to detect many new sources, mainly clusters of galaxies and active galactic nuclei, i.e. accreting black holes. One important avenue for follow-up observations will be the optical data provided by SDSS in its fourth programme (to begin in 2014).


The hardware that the SDSS used to measure distances to the thousands of galaxies in Data Release 9.

The bottom image shows The Sloan Foundation Telescope. The top left image shows the metre-wide aluminium plate that the SDSS uses to measure more than 600 galaxy spectra at once. Each hole in the plate matches the position of a star or galaxy. A fiber optic cable (red and blue in the bottom image) connects each hole to the SDSS spectrograph.

In preparation for this, members of the high-energy group will be using already BOSS data to look at X-ray sources' spectra in order to optimize the synergies once eROSITA data will be available. In particular, three BOSS "ancillary" programmes have been approved recently. They will look at clusters of galaxies, active galactic nuclei over a sparsely populated wide area for angular correlation studies, and sources in a densely populated, smaller area to study the evolution of accreting black holes.


Already today, all the new DR9 images and spectra contain the promise of new discoveries about our Universe -- but SDSS-III is only in the middle of its six-year survey. With the public data release anyone on the Internet can now access and analyse the same data, scientists and laypersons alike.


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