First Light for BOSS: New search for dark energy
|Optical image of one of the approximately 200 quasars captured in the first full focal plane exposure taken by BOSS Courtesy of David Hogg, New York University|
On the night of September 14, 2009, the Baryon Oscillation Spectroscopic Survey (BOSS) achieved first light when it acquired data with an upgraded spectrographic system across the entire focal plane of the Sloan Foundation 2.5-meter telescope at Apache Point Observatory. BOSS is the most ambitious attempt yet to measure the effects of dark energy using the technique known as baryon acoustic oscillation. First exposure was made after many nights of clouds and rain in the Sacramento Mountains when spectroscopy was obtained of some 800 galaxies and 200 quasars in the constellation Aquarius.
The largest program in the Sloan Digital Sky Survey-III, BOSS has 160 participants from among SDSS-III’s 350 scientists and 42 institutions. BOSS’s principal investigator is David Schlegel, its survey scientist is Martin White, and its instrument scientist is Natalie Roe; all three are with the Physics Division at by Lawrence Berkeley National Laboratory. Daniel Eisenstein of the University of Arizona is the director of SDSS-III.
“Baryon oscillation is a fast-maturing method for measuring dark energy in a way that’s complementary to the proven techniques of supernova cosmology,” says Schlegel. “The data from BOSS will be some of the best ever obtained on the large-scale structure of the Universe.”
|One of the BOSS cartridges containing 1,000 optical fibers, which guide light from specific target galaxies and quasars to the spectrograph; Sloan Foundation telescope in background Courtesy of Dan Long, Senior Operations Engineer, Apache Point Observatory|
Measuring baryon oscillations
Baryon oscillations began as pressure waves propagated through the hot plasma of the early universe, creating regions of varying density that can be read today as temperature variations in the cosmic microwave background. The same density variations left their mark as the Universe evolved, in the periodic clustering of visible matter in galaxies, quasars and intergalactic gas, as well as in the clumping of invisible dark matter.
Comparing these scales at different eras makes it possible to trace the details of how the Universe has expanded throughout its history — information that can be used to distinguish among competing theories of dark energy.
BOSS will measure 1.4 million luminous red galaxies at redshifts up to 0.7 (when the Universe was roughly seven billion years old) and 160,000 quasars at redshifts between 2.0 and 3.0 (when the Universe was only about three billion years old). BOSS also will measure variations in the density of hydrogen gas between the galaxies. The observation program will take five years.
“BOSS will survey the immense volume required to obtain percent-level measurements of the BAO scale and transform the BAO technique into a precision cosmological probe,” says survey scientist White. “The high precision, enormous dynamic range, and wide redshift span of the BOSS clustering measurements translate into a revolutionary data set, which will provide rich insights into the origin of cosmic structure and the contents of the Universe.”
Existing SDSS spectrographs were upgraded to include new red cameras more sensitive to the red portion of the spectrum, featuring CCDs designed and fabricated at Berkeley Lab, with much higher efficiency than standard astronomical CCDs in the near infrared.
“Visible light emitted by distant galaxies arrives at Earth redshifted into the near-infrared, so the improved sensitivity of these CCDs allows us to look much further back in time,” says BOSS instrument scientist Roe.
|The spectrum of one of the quasars captured in the BOSS “first light” exposure Courtesy of Bhardwaj, Hogg, Ross|
To make these measurements, BOSS will craft two thousand metal plates to fit the telescope’s focal plane, plotting the precise locations of two million objects across the northern sky. Each morning, astronomers begin plugging optical fibers into a thousand tiny holes in each of the “plug plates” to carry the light from each specific target object to an array of spectrographs.
Glitches met and surmounted
Steering each optical fiber to the right CCD was no trivial task, says Schlegel. “The new BOSS fiber cartridges are snake pits of a thousand fibers each. It would be a disaster if you didn’t know which one went where.”
With a thousand holes in each plug plate, stopping to seek out specific holes to plug a fiber into, or tracing where each fiber ends up, would take an impossibly long time. Instead, a computer assigns the correct target identity to each fiber as a fiber-mapping laser beam moves over the plugged-in fibers and records where the light from each emerges.
Fast and simple — but not quite foolproof. “In our first test images, it looked like we’d just taken random spectra from all over,” Schlegel says. After some hair-pulling, the problem turned out to be simple. “After we flipped the plus and minus signs in the program, everything worked perfectly.”
Now, BOSS is on its way to generating data of unprecedented precision on two million galaxies and quasars, and density variations in the intergalactic gas. The SDSS tradition of releasing data to the public will continue, with the first release from SDSS-III planned for December 2010.
Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy. The extensive upgrade to the SDSS spectrographs and fiber cartridges was supported in large part by competitive grants to the SDSS-III collaboration from DOE’s Office of Science, which will also be helping to support the ongoing operations of BOSS in SDSS-III.
The optical systems were designed and built at Johns Hopkins University, with new CCD cameras designed and built at Princeton University and the University of California at Santa Cruz/Lick Observatory. The University of Washington contributed new optical fiber systems, and Ohio State University designed and built an upgraded BOSS data-acquisition system. The unique “fully depleted” 16-megapixel CCDs for the red cameras evolved from Berkeley Lab research and development on radiation-hard particle detectors for high-energy physics experiments and were fabricated in Berkeley Lab’s MicroSystems Laboratory (MSL).
SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration: the University of Arizona, the Brazilian Participation Group, University of Cambridge, University of Florida, the French Participation Group, the German Participation Group, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, New Mexico State University, New York University, the Ohio State University, the Pennsylvania State University, University of Portsmouth, Princeton University, University of Tokyo, the University of Utah, Vanderbilt University, University of Virginia, and the University of Washington.