3D galaxy map may solve dark matter mystery

NYU / U. PITTSBURGH / JOHNS HOPKINS (US) — Astronomers have constructed the largest-ever 3D map of massive galaxies and distant black holes.

The project will help clarify the mysterious “dark matter” and “dark energy” that make up 96 percent of the universe.

Early last year, the Sloan Digital Sky Survey III (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 the six-year project, begins expansion of the 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,” says 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, so mapping them 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,” says 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,” says 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.”

The 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,” says Ani Thakar of Johns Hopkins University.

And DR9 doubtless contains many surprises.

“This is science at its collaborative best,” says 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.”

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the participating institutions, the National Science Foundation, and the US Department of Energy Office of Science.

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  1. Miguel Iniguez

    What 96 percent of the universe they are talking about?
    Did they measure the all universe already?
    We need to be more cautious and try not to mislead readers with wrong statistics.

  2. Stanley Kerns

    What if (oh dread thought) light, as it ages, increases in wave length–we certainly haven’t observed it long enough to know–maybe it decays.

  3. Onanymous

    @ Stanley Kerns

    If I get your meaning, that’s called Tired Light. It’s been proposed, and has been dismissed as well as, I think, disproved. You can look up Tired Light if you’re interested. But I still have an uninformed layman’s hunch that the answer may be something like that after all.

    I don’t like scientists calling it Dark “Energy,” as it may not be energy at all, and in which case saying it comprises 96% of the Universe would be also wrong. If the expansion isn’t imparting inertial forces to the matter, then is it energy? And I just don’t like the idea of energy increasing over distance and over time. I have no reason to take my preferences seriously, of course.

    My high school math ill equips me to test it, but I like the idea that one property of space is that it expands in the absence of mass (whatever mass is), and contracts in its presence. It would account for the perceived acceleration as distant galaxies got more distant, and for Einstein-like space curvature near masses, and wouldn’t require any twenty-fold increases in the energy, or stuff, of the Universe. I’d think investigating the math of black holes would answer the question.

    They say Dark Energy is weird, and it might be, but Dark Matter seems even weirder to me, because of the apparently spotty distribution of the apparent stuff. Why do some galaxies seem to have it, and others don’t? For what it’s worth, and i think I’m being realistic in my assessment of that, I don’t like that either.

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