Alex Szalay

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The Sloan Digital Sky Survey

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

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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 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 0.05
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Relevant Scales
Distances measured in Mpc megaparsec 1
Mpc 3 x 1024 cm 5 Mpc distance
between galaxies 3000 Mpc scale of the
Universe
if ? gt200 Mpc fluctuations have a PRIMORDIAL
shape if ? lt100 Mpc gravity creates sharp
features, like walls, filaments and voids
Biasing conversion of mass into light is
nonlinear light is much more clumpy than the mass
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The Topology of Local Universe
Measure the Topology of the Universe
Does it consist of walls and voids
or is it randomly distributed?
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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 70
man-years of development effort. (Fermilab
collaboration scientists) Very high data
volume Expect over 20 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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The Footprint of the Survey
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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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Optimal Tiling
Fields have 3 degree diameter Centers determined
by an optimization procedure A total of
2200 pointings 640 fibers assigned simultaneously
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The Mosaic Camera
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Photometric Calibrations
The SDSS will create a new photometric
system u' g' r' i' z' Primary standards
observed with the USNO 40-inch telescope in
Flagstaff Secondary standards observed with
the SDSS 20-inch telescope at Apache
Point calibrating the SDSS imaging data
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The Spectrographs
Two double spectrographs very high
throughput two 2048x2048 CCD detectors
mounted on the telescope light fed through
slithead
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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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Spectrograph Status
Spectrographs Laboratory observations of
solar spectrum First astronomical
observations March 1999
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JHU Contributions
Fiber spectrographs P. FeldmanA. UomotoS.
FriedmanS. Smee
Science Archive A. SzalayA. ThakarP. KunsztI.
CsabaiGy. SzokolyA. ConnollyA. Chaudhaury
Management T. HeckmanT. PoehlerJ. CrockerA.
DavidsenA. UomotoA. SzalayR. Wyse
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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
Three of the four highest redshift quasars have
been found in the first SDSS test data !
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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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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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Who will be using the archive?
Power Users sophisticated, with lots of
resources research is centered around the
archive data moderate number of very intensive
queries mostly statistical, large output
sizes General Astronomy Public frequent, but
casual lookup of objects/regions the archives
help their research, but not central to
it large number of small queries a lot of
cross-identification requests Wide
Public browsing a Virtual Telescope can have
large public appeal need special
packaging could be a very large number of
requests
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How will the data be analyzed?
The data are inherently multidimensional gt
positions, colors, size, redshift Improved
classifications result in complex N-dimensional
volumes gt complex constraints, not
ranges Spatial relations will be
investigated gt nearest neighbors gt other
objects within a radius Data Mining finding the
needle in the haystack gt separate typical
from rare gt recognize patterns in the
data Output size can be prohibitively large for
intermediate files gt import output directly
into analysis tools
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Geometric Approach

• The Main Problem
• fast, indexed, complex searches of Terabytes in
  k-dim space
• searches are not necessary parallel to the
  axes gt traditional indexing (b-tree) does not
  work

• Geometric Approach
• Use the geometric nature of the k-dimensional
  data
• Quantize data into containers of
  friends objects of similar colors close on
  the sky stored together gt efficient cache
  performance
• Containers represent a coarse grained density map
  of the data multidimensional index tree k-d
  tree r-tree

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Organization of Searches
Queries are inherently geometric the primitive
constraint is a half-space formed by a linear
combination gt k-dimensional hyperplane Boolean
combinations are allowed the constraints form
k-dimensional polyhedra Queries are run on the
coarse grained map determine intersections of
index tree and query polyhedron List of
containers is prepared for query projections of
full query time and output volume created The
list of containers and query is sent to the
Search Engine actual searches quantized by
containers Searches can be optimized, executed
in parallel
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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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Sky coordinates
Stored as Cartesian coordinates projected onto
a unit sphere Longitude and Latitude
lines intersections of planes and the
sphere Boolean combinations query polyhedron
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Sky Partitioning
Hierarchical Triangular Mesh - based on octahedron
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Hierarchical Subdivision
Hierarchical subdivision of spherical
triangles represented as a quadtree In SDSS the
tree is 5 levels deep - 8192 triangles
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Result of the Query
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Magnitudes and Multicolor Searches

• Galaxy fluxes
• large dynamic range
• errors
• divergent as x? 0 !

For multicolor magnitudes the error
contours can be very anisotropic and
skewed, extremely poor localization!
But this is an artifact of the logarithm at zero
flux, in flux space the object is well localized
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Novel Magnitude Scale
b softnessc set to match normal magnitudes

• Advantages
• monotonic
• degrades gracefully
• objects have small error ellipse
• unified handling of detections and upper
  limits!
• Disadvantages
• unusual
• (Lupton, Gunn and Szalay, AJ 99)

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Flux Indexing
Split along alternating flux directions Create
balanced partitions Store bounding boxes at each
stepBuild a 10-12 level tree in each triangle
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How to build compact cells?
The SDSS will measure fluxes in 5 bands gt
asinh magnitudes Axis-parallel splits in median
flux, in 8 separate zones in Galactic
latitude gt 5 dimensional bounding boxes
The fluxes are strongly correlated gt 2 ?
dimensional distribution of typical objects gt
widely scattered rare objects gt large density
contrasts
Therefore first create a local density and
split on its value (Csabai etal 96) typical
(98) rare (2)
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Coarse Grained Design

Archive
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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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Exploring new methods
New spectral classification techniques galaxy
spectra can be expressed as a superposition of a
few (lt5) principal components gt objective
classification of 1 million spectra!
Photometric redshifts galaxy colors
systematically change with redshift, the SDSS
photometry works like a 5-pixel spectrograph gt
?z0.05, but with 100 million objects!
Measuring cosmological parameters before data
analysis was limited by small number
statistics after dominant errors are systematic
(extinction) gt new analysis methods are
required!
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Photometric redshifts
Multicolor photometry maps physical
parameters luminosity L redshift z
spectral type T
Inversion u,g,r,I,z gt z, L,
T
observed fluxes
Redshifts are statistical, with large errors
?z?0.05 The data set is huge, more than 100
million galaxies Easy to subdivide into coarse z
bins, and by type gt study evolution gt
enormous volume - 1 Gpc3
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Spectra from Photometry
New development low resolution spectra from
multicolor photometry many galaxies gt
oversampling gt spectra (Csabai, Budavari,
Connolly, Szalay 99)
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Measuring P(k)
Karhunen-Loeve transform Signal-to-noise
eigenmodes of the redshift survey Optimal
extraction of clustering signal Maximal
rejection of systematic errors(Vogeley and
Szalay 96, Matsubara, Szalay and Landy 99)
Pilot project using the Las Campanas Redshift
Survey with 22,000 galaxies
We simultaneously measure the values of the
redshift-distortion parameter (??0.6/b),
the normalization (?8 ) and the CDM shape
parameter ( ? ?h).
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The SDSS Science Archive

• The SDSS Science Archive will support complex
  multidimensional queries of Terabyte catalogs
• Queries are primarily I/O limited
• distributed parallel architecture
• Our usage patterns required a fast
  multidimensional index
• sky positions
• object colors
• Advanced query capabilities required
• spatial relations
• classifications
• condensed representations
• Soon other large astronomical archives will
  emerge
• cross-referencing
• seamless interoperability
• New paradigm in Astronomy

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Future of Archives
The next generation of astronomical archives with
TB catalogs will dramatically change
astronomy top-down design large sky
coverage built on sound statistical
plans uniform, homegeneous, well
calibrated well controlled and documented
systematics The technology to store and index
the data is here 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
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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 store and index
the data is here 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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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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