New 3D map of massive galaxies and black holes offers clues to dark matter, dark energy

The map was produced by the Sloan Digital Sky Survey III (SDSS-III).

Early last year, the SDSS-III released the largest-ever image of the sky, which covered one-third of the night sky. The new data, "Data Release 9" (DR9), which publicly releases the data from the first two years of this six-year project, begins expansion of this earlier image into a full three-dimensional map.

"What really makes me proud of this survey is our commitment to creating a legacy for the future," said Michael Blanton, a New York University physics professor who led the team that prepared DR9. "Our goal is to create a map of the universe that will be used long after we are done, by future generations of astronomers, physicists, and the general public."

DR9 is the latest in a series of data releases stretching back to 2001. This release includes new data from the ongoing SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), which will eventually measure the positions of 1.5 million massive galaxies over the past seven billion years of cosmic time, as well as 160,000 quasars -- giant black holes actively feeding on stars and gas -- from as long ago as 12 billion years in the past.

BOSS is targeting these big, bright galaxies because they live in the same places as other galaxies and they're easy to spot, even far away in the universe. Mapping these big galaxies thus provides an effective way to make a map of the rest of the galaxies in the universe.

With such a map, scientists can retrace the history of the universe over the last seven billion years. With that history, they can get better estimates for how much of the universe is made up of "dark matter" -- matter that we can't directly see because it doesn't emit or absorb light -- and "dark energy," the even more mysterious force that drives the accelerating expansion of the universe.

"Dark matter and dark energy are two of the greatest mysteries of our time," said David Schlegel of Lawrence Berkeley National Laboratory, who led the SDSS-III effort to map these galaxies and quasars. "We hope that our new map of the universe can help someone solve the mystery."

That map of the universe is the centerpiece 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 expansion, are combined by SDSS-III scientists to create the three-dimensional map released with DR9.

Distant "quasars" provide another way to measure the distribution of matter in the universe. Quasars 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 Earth.

These new data are not only helping us understand the distant universe, but also our own cosmic backyard, the Milky Way galaxy. DR9 includes better estimates for the temperatures and chemical compositions of more than half a million stars in our own galaxy.

"With these better estimates, we can look back at the history of our galaxy," said Connie Rockosi of the University of California, Santa Cruz, who leads the SDSS-III's Milky Way study. "We can tell the story of how smaller galaxies came together to build up the Milky Way we see today."

All these new images and spectra contain the promise of new discoveries about our universe -- but the SDSS-III is only in the middle of its six-year survey.

"The most fun part of making this data available online is knowing that anyone on the Internet can now access the very same data and search tools that professional astronomers use to make exciting discoveries about our universe," said Ani Thakar of Johns Hopkins University.

And DR9 doubtless contains many surprises.

"This is science at its collaborative best," said Michael Wood-Vasey, a professor at the University of Pittsburgh and the scientific spokesperson for the SDSS-III collaboration. "SDSS-III scientists work together to address big questions extending from our own galaxy to distant reaches of the universe and then they share all of that data with the world to allow anyone to make the next big discovery."