Led
by Berkeley Lab scientists, the Sloan Digital Sky Survey’s BOSS is bigger than
all other spectroscopic surveys combined for measuring the universe’s
large-scale structure
The Third Sloan Digital Sky Survey
(SDSS-III) has issued Data Release 9 (DR9), the first public release of data
from the Baryon Oscillation Spectroscopic Survey (BOSS). In this release BOSS,
the largest of SDSS-III’s four surveys, provides spectra for 535,995 newly
observed galaxies, 102,100 quasars, and 116,474 stars, plus new information
about objects in previous Sloan surveys (SDSS-I and II).
“This is just the first of three data
releases from BOSS,” says David Schlegel of the U.S. Department of Energy’s
Lawrence Berkeley National Laboratory (Berkeley Lab), an astrophysicist in the
Lab’s Physics Division and BOSS’s principal investigator. “By the time BOSS is
complete, we will have surveyed more of the sky, out to a distance twice as
deep, for a volume more than five times greater than SDSS has surveyed before –
a larger volume of the universe than all previous spectroscopic surveys
combined.”
Spectroscopy yields a wealth of
information about astronomical objects including their motion (called redshift
and written “z”), their composition, and sometimes also the density of the gas
and other material that lies between them and observers on Earth. The BOSS
spectra are now freely available at http://sdss3.org to a public that includes
amateur astronomers, astronomy professionals who are not members of the
SDSS-III collaboration, and high-school science teachers and their students.
The new release lists spectra for
galaxies with redshifts up to z = 0.8 (roughly 7 billion light years away) and
quasars with redshifts between z = 2.1 and 3.5 (from 10 to 11.5 billion light
years away). When BOSS is complete it will have measured 1.5 million galaxies
and at least 150,000 quasars, as well as many thousands of stars and other
“ancillary” objects for scientific projects other than BOSS’s main goal.
The
key to the history of the universe
BOSS is designed to measure baryon
acoustic oscillation (BAO), the large-scale clustering of matter in the
universe. BAO began as rippling fluctuations (“sound waves”) in the hot, dense
soup of matter and radiation that made up the early universe. As the universe
expanded it cooled. Finally atoms formed and radiation went its own way; the
density ripples left their marks as temperature variations in the cosmic
microwave background (CMB), where they can be detected today.
The CMB came into being 380,000 years
after the big bang, over 13.6 billion years ago, and continues to stretch
across the entire sky as the universe expands. Peaks in CMB temperature
variation occur about half a billion light years apart, at the same angle,
viewed from Earth, as peaks in the large-scale galactic structure that evolved
billions of years later. The regions of higher density in the CMB were in fact
the sources of galaxy formation; they correspond to regions where galaxies
cluster, along with intergalactic gas and concentrations of much more massive
underlying dark matter. The natural “standard ruler” marking peaks in
clustering can be applied not only across the sky but in all three dimensions,
backward in time to the CMB.
Distant quasars provide another way of
measuring BAO and the distribution of matter in the universe. Quasars are the
brightest objects in the distant universe, whose spectra bristle with
individually shifted absorption lines, a “Lyman-alpha forest” unique to each
that reveals the clumping of intergalactic gas and underlying dark matter
between the quasar and Earth.
Marks
on the cosmic ruler
Schlegel has called BAO “an
inconveniently sized ruler,” requiring “a huge volume of the universe just to
fit the ruler inside,” but it’s a precision tool for tracking the universe’s
expansion history, and for probing the nature of gravity and the mysterious
dark energy that’s causing expansion to accelerate.
To fill the huge volume, BOSS had to
find more and fainter objects in the sky at greater distances than SDSS had
attempted before. The camera system and spectrographs of the 2.5-meter Sloan
Foundation Telescope at the Apache Point Observatory in New Mexico had to be
completely rebuilt.
SDSS uses “plug plates” at the
telescope’s focal plane, aluminum disks with holes drilled to match the precise
position of previously imaged target objects. SDSS-I and II plug plates had
only 640 holes apiece, each covering three arcseconds; BOSS is using 2,000 plug
plates with 1,000 holes apiece, each covering a tight two arcseconds to reduce
light that’s not from the target.
Optical fibers are plugged into the
holes every day by hand, to guide the light from each target to a spectrograph.
While weather conditions vary night to night, observations on the best nights
use up to nine plug plates. For BOSS, the spectrographs were rebuilt with new
optics and new CCD detectors designed and fabricated at Berkeley Lab.
“Light from distant galaxies arrives at
Earth redshifted into the infrared,” says Natalie Roe, director of Berkeley Lab’s
Physics Division and BOSS’s instrument scientist, who led construction of the
spectrographs. “We optimized the BOSS spectrographs for mapping exactly these
galaxies.”
Working with Schlegel and Adam Bolton at
the University of Utah, Berkeley Lab’s Stephen Bailey is in charge of daily
“extraction pipeline” operations that convert raw data from the telescope into
useful spectra and quantities derived from them, ready for scientific analysis.
Data storage and the extraction pipeline run on the Riemann Linux cluster of
Berkeley Lab’s High-Performance Computing Services Group; the data is copied
from Riemann to the University of Utah, New York University, Johns Hopkins
University, and the National Energy Research Scientific Computing Center
(NERSC) at Berkeley Lab. The Lab also hosts the SDSS-III website,
http://sdss3.org, from which the data can be downloaded.
“Data releases are a proud tradition for
SDSS, and the first BOSS data greatly increase the SDSS store of information,”
Bailey says. “Members of the SDSS-III collaboration get first crack at it –
with barely enough time to write up their results – but three times as many
papers based on the data are published by scientists outside the
collaboration.”
Says Schlegel, “SDSS-III is already the
most used of all surveys from any telescope in the world, including the Keck
telescopes and the Hubble Space Telescope. With DR9, BOSS contributes a huge
information increase for all kinds of scientific investigations, from quasars to
how stars evolve to really odd objects like galaxy-scale strong gravitational
lenses. Meanwhile the BOSS BAO survey is over two-thirds finished, and ahead of
schedule – we’re well on our way to the best measure of BAO that will be made
for a long time. All the data BOSS collects will be available to anyone who can
use it.”
###
“The Ninth Data Release of the Sloan
Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon
Oscillation Spectroscopic Survey,” by Christopher Ahn et al, has been submitted
to the Astronomical Journal and may be found on the arXiv preprint server at
http://arxiv.org/abs/1207.7137.
“The Baryon Oscillation Spectroscopic
Survey of SDSS-III,” by Kyle Dawson, David Schlegel, and members of the BOSS
Collaboration, has been submitted to the Astronomical Journal and may be found
on the arXiv preprint server at http://arxiv.org/abs/1208.0022.
“Spectral Classification and Redshift
Measurement for the SDSS-III Baryon Oscillation Spectroscopic Survey,” by Adam
Bolton et al, has been submitted to the Astronomical Journal and may be found
on the arXiv preprint server at http://arxiv.org/abs/1207.7326
References to these and other papers
relating to Data Release 9 are in the SDSS-III Collaboration release at
http://www.sdss3.org/press/. Berkeley Lab researchers who are members of BOSS
and contributed to these papers include Stephen Bailey, William Carithers,
Andreu Font-Ribera, Jessica Kirkpatrick, Beth Reid, Natalie Roe, Nicholas Ross,
David Schlegel, and Martin White.
Lawrence Berkeley National Laboratory
addresses the world’s most urgent scientific challenges by advancing
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revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s
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science.energy.gov/.
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 Office of Science. The
SDSS-III web site is http://www.sdss3.org.
SDSS-III is managed by the Astrophysical
Research Consortium for the Participating Institutions of the SDSS-III
Collaboration including the University of Arizona, the Brazilian Participation
Group, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon
University, University of Florida, the French Participation Group, the German
Participation Group, Harvard University, the Instituto de Astrofisica de
Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins
University, Lawrence Berkeley National Laboratory, Max Planck Institute for
Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico
State University, New York University, Ohio State University, Pennsylvania
State University, University of Portsmouth, Princeton University, the Spanish
Participation Group, University of Tokyo, University of Utah, Vanderbilt
University, University of Virginia, University of Washington, and Yale
University.
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