Alex Szalay

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The Sloan Digital Sky Survey

• Alex Szalay
• Department of Physics and Astronomy
• The Johns Hopkins University
• and the SDSS Project

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The Sloan Digital Sky Survey
A project run by the Astrophysical Research
Consortium (ARC)
The University of Chicago Princeton
University The Johns Hopkins University The
University of Washington Fermi National
Accelerator Laboratory US Naval Observatory
The Japanese Participation Group The Institute
for Advanced Study Max Planck Inst,
Heidelberg SLOAN Foundation, NSF, DOE, NASA
Goal To create a detailed multicolor map of the
Northern Sky over 5 years, with a budget of
approximately 80M Data Size 40 TB raw, 1 TB
processed
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Scientific Motivation
Create the ultimate map of the Universe ? The
Cosmic Genome Project! Study the distribution of
galaxies ? What is the origin of
fluctuations? ? What is the topology of the
distribution? Measure the global properties of
the Universe ? How much dark matter is
there? Local census of the galaxy population ?
How did galaxies form? Find the most distant
objects in the Universe ? What are the highest
quasar redshifts?
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Cosmology Primer
The Universe is expanding the galaxies move
away from us spectral lines are redshifted
v Ho r Hubbles law
The fate of the universe depends on the
balance between gravity and the expansion
velocity
? density/criticalif ? lt1, expand forever
Most of the mass in the Universe is dark
matter, and it may be cold (CDM)
?dgt ?
The spatial distribution of galaxies is
correlated, due to small ripples in the early
Universe
P(k) power spectrum
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The Naught Problem
What are the global parameters of the
Universe? H0 the Hubble constant 55-75
km/s/Mpc ?0 the density parameter 0.25-1 ?0 the
cosmological constant 0 - 0.7 Their values are
still quite uncertain today... Goal measure
these parameters with an accuracy of a few percent
High Precision Cosmology!
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The Cosmic Genome Project
The SDSS will create the ultimate mapof the
Universe, with much more detailthan any other
measurement before
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Area and Size of Redshift Surveys
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Clustering of Galaxies
We will measure the spectrum of the density
fluctuations to high precision even on very
large scales
The error in the amplitude of the
fluctuation spectrum 1970 x100 1990 x2 1995
0.4 1998 0.2 1999 0.1 2002 0.02
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Finding the Most Distant Objects
Intermediate and high redshift QSOs
Multicolor selection function.
Luminosity functions and spatial clustering.
High redshift QSOs (zgt5).
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Features of the SDSS
Special 2.5m telescope, located at Apache Point,
NM 3 degree field of view. Zero distortion
focal plane. Two surveys in one Photometric
survey in 5 bands. Spectroscopic redshift
survey. Huge CCD Mosaic 30 CCDs 2K x
2K (imaging) 22 CCDs 2K x 400 (astrometry) Two
high resolution spectrographs 2 x 320 fibers,
with 3 arcsec diameter. R2000 resolution with
4096 pixels. Spectral coverage from 3900Å to
9200Å. Automated data reduction Over 100
man-years of development effort. (Fermilab
collaboration scientists) Very high data
volume Expect over 40 TB of raw data. About 1
TB processed catalogs. Data made available to
the public.
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Apache Point Observatory
Located in New Mexico, near White Sands National
Monument
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The Telescope
Special 2.5m telescope 3 degree field of
view Zero distortion focal plane Wind
screen moved separately
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The Photometric Survey
Northern Galactic Cap 5 broad-band filters
( u', g', r', i', z )
limiting magnitudes (22.3, 23.3, 23.1, 22.3,
20.8) drift scan of 10,000 square degrees
55 sec exposure time 40 TB raw imaging
data -gt pipeline -gt 100,000,000 galaxies
50,000,000 stars calibration to 2 at
r'19.8 only done in the best seeing (20
nights/yr) pixel size is 0.4 arcsec,
astrometric precision is 60 milliarcsec Southern
Galactic Cap multiple scans (gt 30 times) of
the same stripe Continuous data rate of 8
Mbytes/sec
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Survey Strategy
Overlapping 2.5 degree wide stripes Avoiding the
Galactic Plane (dust) Multiple exposures on the
three Southern stripes
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The Spectroscopic Survey
Measure redshifts of objects ? distance SDSS
Redshift Survey 1 million galaxies 100,000
quasars 100,000 stars Two high throughput
spectrographs spectral range 3900-9200 Å. 640
spectra simultaneously. R2000
resolution. Automated reduction of spectra Very
high sampling density and completeness Objects in
other catalogs also targeted
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The Mosaic Camera
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The Fiber Feed System
Galaxy images are captured by optical fibers
lined up on the spectrograph slit Manually
plugged during the day into Al plugboards 640
fibers in each bundle The largest fiber system
today
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First Light Images
Telescope First light May 9th 1998
Equatorial scans
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The First Stripes
Camera 5 color imaging of gt100 square
degrees Multiple scans across the same
fields Photometric limits as expected
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NGC 2068
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UGC 3214
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NGC 6070
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The First Quasars
The four highest redshift quasars have been
found in the first SDSS test data !
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Methane/T Dwarf

• Discovery of several newobjects by SDSS 2MASS

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Detection of Gravitational Lensing
28,000 foreground galaxies and 2,045,000
background galaxies in test data(McKay etal 1999)
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SDSS Data Flow
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Data Processing Pipelines
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Concept of the SDSS Archive
Science Archive (products accessible to users)
OperationalArchive (raw processed data)
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Distributed Collaboration
Fermilab
U.Chicago
U.Washington
ESNET
I. AdvancedStudy
Japan
Princeton U.
VBNS
JHU
Apache PointObservatory
USNO
NMSU
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SDSS Data Products
Object catalog 400 GB parameters of
gt108 objects Redshift Catalog 1 GB
parameters of 106 objects Atlas Images 1.5
TB 5 color cutouts of gt108 objects
Spectra 60 GB in a one-dimensional
form Derived Catalogs 20 GB - clusters
- QSO absorption lines 4x4 Pixel All-Sky Map
60 GB heavily compressed
All raw data saved in a tape vault at Fermilab
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Geometric Indexing
Divide and Conquer
Partitioning
3 ? N ? M
HierarchicalTriangular Mesh
Split as k-d treeStored as r-treeof bounding
boxes
Using regularindexing techniques
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Distributed Implementation
User Interface
Analysis Engine
Master
SX Engine
Objectivity Federation
Objectivity
Slave
Slave
Slave
Objectivity
Slave
Objectivity
Objectivity
RAID
Objectivity
RAID
RAID
RAID
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Collaboration with Particle Physics

• Collaboration with the Analysis Data Grid
• proposal to the NSF KDI program by
• JHU, Fermilab and Caltech (H. Newman, J. Bunn)
• Objectivity, Intel and Microsoft (Jim Gray)
• Involves computer scientists, astronomers and
  particle physicists
• Accessing Large Distributed Archives in Astronomy
  and Particle Physics
• experiment with scalable server architectures,
• create middleware of intelligent query agents,
• apply to both particle physics and astrophysics
  data sets
• Status
• 3 year proposal just funded

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The Age of Mega-Surveys
The next generation of astronomical archives with
Terabyte catalogs will dramatically change
astronomy top-down design large sky
coverage built on sound statistical
plans uniform, homogeneous, well
calibrated well controlled and documented
systematics The technology to acquire, store and
index the data is here we are riding Moores
Law Data mining in such vast archives will be a
challenge, but possibilities are quite
unimaginable Integrating these archives into a
single entity is a project for the whole
community gt Virtual National Observatory
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New Astronomy Different!

• Systematic Data Exploration
• will have a central role in the New Astronomy
• Digital Archives of the Sky
• will be the main access to data
• Data Avalanche
• the flood of Terabytes of data is already
  happening, whether we like it or not!
• Transition to the new
• may be organized or chaotic

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NVO The Challenges

• Size of the archived data
• 40,000 square degrees is 2 trillion pixels
• One band 4 Terabytes
• Multi-wavelength 10-100 Terabytes
• Time dimension few Petabytes
• The development of
• new archival methods
• new analysis tools
• new standards (metadata, interchange formats)
• Hardware/networking requirements
• Training the next generation!

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Summary
The SDSS project combines astronomy, physics, and
computer science
It promises to fundamentally change our view of
the universe
It will determine how the largest structures in
the universe were formed
It will serve as the standard astronomy
reference for several decades
Its virtual universe can be explored by both
scientists and the public
Through its archive it will create a new paradigm
in astronomy
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