Sloan Digital Sky Survey: from the Coolest Brown Dwarfs to the Most Distant Quasars

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Sloan Digital Sky Survey from the
Coolest Brown Dwarfs to the Most Distant
Quasars

• Xiaohui Fan
• Institute for Advanced Study, Princeton

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Revolution in Astronomical Observations

• Digital Detector Technology
• Charge-coupled devices (CCDs) in optical and
  X-ray
• Large IR arrays
• Large ground-based and Space Telescopes
• New 8-10 meter class telescopes and telescope
  arrays
• Hubble Space Telescope, Chandra X-ray
  Observatory, SIRTF
• New Generation of Digital Sky Surveys in optical
  and near IR wavelengths
• Complete census of the sky Tera-bytes databases
• Statistical studies
• Rarest objects on the sky

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Outline

• Sloan Digital Sky Survey Project
• Science Goals
• Technical challenges
• Survey status and early science results
• Search for the Reddest Objects on the Sky
• Discovery of the most distant quasars
• Discovery of the coolest and smallest brown
  dwarfs
• Probing the End of Cosmic Dark Ages
• Discovery of the first complete Gunn-Peterson
  Trough
• Evolution of the ionization state of the
  intergalactic medium
• Epoch of reionization

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Sloan Digital Sky Survey
A Wide-Area, Deep, Multi-Object Imaging
and Spectroscopic Survey in Optical Wavelength

• Main Imaging Survey
• 10,000 square degrees of the North Galactic
  Cap
• 100 million 5-band images
• limiting magnitude of r23
• photometric errors lt 2

• Spectroscopic Survey
• brightest 1 million galaxies
• brightest 100,000 quasars
• ? Redshift survey, 3-D distribution

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SDSS Hardware and Software

• 2.5m Dedicated Telescope
• Ritchey-Chretien design with 3o field-of-view
• f/2.2 primary with an image scale of 0.4
  arcsec/24 micron pixel
• Imaging Camera
• 30 primary 2048x2048 CCDs
• 24 2048x400 astrometric and focus CCDs
• Filters ugriz (3000-11000 A)
• Drift-scan mode 55 sec exposures --gt
  limiting magnitude r 23
• Data rate 15 deg/hr 5MB/s 170 GB/night 12
  TB total
• Spectrographs
• 2 fiber-fed (3 arcsec diameter) double
  spectrographs
• 640 fibers at once, using pre-drilled plug
  plates.
• Wavelength 3800-9200 Angstroms Resolution
  2000
• Photometric Telescope
• Fully Automated 0.5-m Telescope
• Establishes photometric system monitors sky
  conditions

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Apache Point Observatory
ARC 3.5m Telescope
SDSS 0.5m Telescope
SDSS 2.5m Telescope
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SDSS 2.5m Telescope
Light and Wind baffle
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SDSS Imaging Camera Top to bottom g z
u i r
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Transmission of Filters in Photometric Camera
Reddest filter Sensitive to quasars And brown
dwarfs
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What does the data actually look like?
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Plugging Spectroscopic Plates
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Survey Participants
Participating Institutions 100 Scientists
Princeton University University of
Chicago Fermilab Institute for Advanced
Study Johns Hopkins University University of
Washington U.S. Naval Observatory Japanese
Participation Group New Mexico State
University Max-Planck A and IA
Funding 100 Million
Alfred P. Sloan Foundation Member
Institutions National Science Foundation
(NSF) National Aeronautics and Space
Administration (NASA) United States Department of
Energy (DOE) Japanese Monbukagakusho The Max
Planck Society
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The SDSS Collaboration
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SDSS Data Releases

• Initial proprietary period
• Public releases every 18 months
• Public release carried out by STScI
• June 2001 Early Data Release (aka the EDR)

500 square degrees of 5-band imaging
50,000 object spectra (redshifts)
(45,000 galaxies, 5000 quasars)
Next data release scheduled for January 2003
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SDSS Project History

• 1988 Project Discussions Begin at OHare
  Meeting
• May 1998 Telescope First Light Imaging
• May/June 1999 Spectroscopic First Light
• Sept/Oct. 1999 --First off-equatorial
  drift-scan imaging
• --First calibrated spectroscopy

• April 2000 Survey begins 5 years to completion
  (104 sq.deg.)
• Feb 2002 3,000 square degrees of 5-band
  imaging
• 250,000 object
  spectra (redshifts)

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SDSS Early Science Results

• Solar System
• Density and color distribution of asteroids
• Stars
• Discovery and classification of brown dwarfs
• Peculiar white dwarfs
• Variable stars
• Galactic Structure
• Halo models
• Tidal streamers

• Galaxies
• Large scale structure
• Weak lensing
• Quasars
• Luminosity function and its evolution
• High-z quasars
• Reionization
• Serendipity discoveries
• Supernovae
• Unusual quasars
• GRBs

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SDSS Galaxy Survey Progress
Goal -- 1,000,000 galaxy in 10,000
deg2 -- z lt 0.3 Progress 250,000
redshifts measured Science -- Luminosity
Func. -- 3-D power spectrum --
density-morphology -- stellar pop
Blanton et al.
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SDSS High-redshift Quasar Survey

• Quasars
• Active nuclei of galaxies
• Most luminous and distant object
• Powered by supermassive black hole (109 Msun)
• Probes of the evolution of the universe
• The Study of High-Redshift Quasars Probes
• The epoch of the first generations of
  galaxies/quasars
• Models of black hole formation
• Role of quasar/BH activity in galaxy evolution
• State of intergalactic medium
• Ionizing background at high-z
• History of reionization ? probing the end of
  cosmic dark ages, when the first stars were
  forming, and the universe was transformed from
  neutral to ionized

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How to Find Quasars in SDSS

• Goal establish a color-selected, flux-limited
  sample of 100,000 quasars in the 10,000 deg2
  survey area
• For quasars at zlt5.5, we have developed fully
  automated pipeline to select quasars
  candidates, take their spectra and measure their
  redshifts
• Step 1 measure the magnitudes of each object in
  the five SDSS passbands colors of the object
• Step 2 select quasar candidates objects with
  quasar-like colors ? quasars and stars have
  very difficult colors different locations in the
  multidimensional color-space
• Step 3 obtain spectra of all the quasar
  candidates
• Step 4 measure the redshift and distance of each
  object

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Color-Color Diagrams
Stellar locus
Z3
Z4
quasar
Z5

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SDSS Quasar Survey Progress

• 20,000 Quasars at 0 lt z lt 6.3
• 200 quasars at zgt4
• First detection of quasars at
• zgt5 and zgt6

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Search for the First Quasars
Fan, Narayanan, Lupton, Strauss et al.

• Color selection of i-drop
  out quasars
• At zgt5.5, Lya enters z-band ? quasars have only
  red i-z measurement faint objects z-band only
  detections
• Technical Challenges
• Rare objects ? contaminant elimination
• Elimination of false z-band only detections ?
  improved cosmic ray rejection
• Reliability of faint z photometry ? follow-up
  high S/N z photometry
• Major contaminants are L and T type Brown Dwarfs
  ? additional IR photometry

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Discovery of T Dwarf

• Red color similar to zgt6 quasars
• Spectrum dominated by water and methane
  absorption
• T 900 K, Mass 20 60 Jupiter masses,
  distance 10pc
• Proto-type Brown dwarf objects intermediate
  between stars and planets, not powered by H
  burning

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Selection Procedures
VLT (Chile) 8m
APO 3.5m
Calar Alto (Spain) 3.5m
Keck (Hawaii) 10m

• SDSS database
• 15 million objects

UKIRT (Hawaii) 3.8m
4. Detailed spectra (4 quasars 5.7 lt z lt 6.3)
2..Photometric pre-selection 150 objects
3. Photometric and spectroscopic Identification
(20 objects)
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Search for the First Quasars Results

• 1500 square degrees of the sky searched
• 6 telescopes, 10 different instruments used
• Spent 2 years, 30 nights of observing time
• 4 most distant quasars discovered
• z5.80, 5.82, 5.99, and 6.28
• By-product of the survey large numbers of brown
  dwarfs discovered

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Brown Dwarfs in SDSS

Discoveries

• identified from their red colors
• gt 100 L dwarfs (1300 lt T lt 1900 K)
• 30 T dwarfs ( T lt 1300 K)
• spectra dominated by methane
• coolest and smallest free-floating
• objects
• mass 20 - 50 Jupiter masses

Sciences

• spectral classification and
• temperature sequence
• atmosphere physics, dust cloud
• spatial density, luminosity function
• and mass density of BDs
• relation between star/BD/planet
• formation

Geballe, Leggett, Knapp Fan, Gilimoski et al.
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Zgt5.7 quasars from the SDSS
Z5.80
Z5.82
Z5.99
Z6.28
Fan et al. 2000, 2001
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Becker, Fan, White et al. 2001

Ly ßOVI
Keck/ESI Spectra of Zgt5.7 Quasars
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Quest for Gunn-Peterson Trough

• Gunn-Peterson (1965) effect
• If the universe was neutral opaque to the quasar
  light at wavelength lower than Lyman alpha
• Create a absorption TROUGH in the quasar spectrum
• Detection of Gunn-Peterson trough signals that we
  have reached the cosmic dark-age and the epoch
  of the first generation galaxy and quasar
  formation
• One of the longest-sought predictions of
  cosmology, but never detected until

Lya
No G-P trough (still flux detected)
G-P trough
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reionization
From Avi Loeb
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Increasing Lya absorption with redshift
zabs fobs/fcon -------------------------
-- 5.5 0.10 5.7
0.05 6.0 lt0.006 Zero flux over
300Å immediately blueward of Lya emission
in z6.28 quasar ? Detection of complete
Gunn-Peterson Trough tgtgt1 over large
region of IGM
Becker, Fan, White et al.
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VLT/FOS2
Detection of A Complete Gunn-Peterson Trough
VLT observation
T-0.0010.003
Pentericci et al.
Pentericci, Fan, Rix et al. 2001
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Evolution of Ionization State of the
Intergalactic Medium

• IGM is highly non-uniform
• Regions with different density have different
  Lya transmission
• To constrain ionization state have to take
  into account the density distributions of the IGM
• IGM properties estimated by comparing with
  cosmological simulations of Lyman absorption in a
  LCDM model
• Four parameters calculated
• Ionizing background ? integrated radiation from
  starlight
• IGM neutral fraction ? how neutral the IGM is
• Critical overdensity ? the minimum density in the
  IGM to be ionized as ionization front progresses
  from high to low density regions
• Mean free path of ionizing photons ? how
  transparent IGM is

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Constraining the Reionization Epoch

• Ionizing background
• Ionizing background declines by a factor of gt25
  from z3 to z6 Indication of a sudden change at
  z6?
• Neutral hydrogen fraction
• Volume-averaged HI fraction increased by gt100
  from z3 to z6
• Mass-averaged HI fraction gt 1
• Universe quickly approaching neutrality at early
  epoch at z6

Mass-averaged
Volume-averaged
Fan et al. 2001
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Constraining Reionization Epoch

• Critical overdensity ?i
• Regions with ? gt ?i remain neutral
• Moderately dense regions in the IGM were still
  neutral at z6
• Mean free path of ionizing photons
• Universe quickly becomes transparent with time
• At z6
• Last remaining neutral regions are being ionized
• Marks the end of reionization epoch??

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Three stages
Pre-overlap
Overlap
Post-overlap
From Haiman Loeb
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Neutral fraction
Ionizing background
Gnedin 2000
Gas density
Gas temperature
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Neutral fraction
Ionizing background
Gas density
Gas temperature
Gnedin 2000
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Mass-averaged
postoverlap
Reionization epoch
Pre- overlap
Volume-averageed
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Significance of the Discovery of Gunn-Peterson
Effect

• The hydrogen in the universe was approaching
  neutral
• The radiation background at z6 is much lower
  than at low redshift
• We are approaching to the epoch of first lights
  in the universe (after the Big Bang) and the end
  of the cosmic dark ages
• These quasars are themselves among the sources of
  first light
• Consistent with one of the fundamental
  predications of cosmology gravitational
  instability produced first stars and galaxies and
  quasars at z 6 10 and re-ionized the universe

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Highest redshift quasars reionization, And the
end of cosmic dark ages
Coolest brown dwarfs