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

Astronomers have constructed the largest-ever three-dimensional map of massive galaxies and distant black holes, which will help the investigation of the mysterious dark matter and dark energy that make up 96 percent of the universe.

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 7 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.

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With such a map, scientists can retrace the history of the universe over
the past 7 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 the 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.”