The HighestRedshift Quasars and the End of Cosmic Dark Ages

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The Highest-Redshift Quasars andthe End of
Cosmic Dark Ages

• Xiaohui Fan
• CollaboratorsStrauss,Schneider,Richards,
  Hennawi,Gunn,Becker,White,Rix,Pentericci, Walter,
  Carilli,Cox,Bertoldi,Omont,Brandt, Vestergaard,
  Jiang, Diamond-Stanic, et al. SDSS collaboration

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questions

• How to discover the most distant quasars in the
  Universe?
  
• When did the earliest quasars and super-massive
  black holes appear in the Universe?
  
• How were the cosmic dark ages ended by the
  first generation of galaxies and quasars?

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End of cosmic dark ages

• Hot Big Bang

• Cosmic Dark Ages no light
• no star, no quasar, universe dark
• IGM atomic (neutral) and opaque
• to UV

• First light the first galaxies
• and quasars in the universe

• End of cosmic dark ages
• Universe lit up and heated up
• Dark -- light
• Neutral -- ionized (reionization)

? today
Courtesy G. Djorgovski
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Why Distant Quasars?

• Existence of supermassive black holes (BHs) at
  the end of cosmic dark ages
  
• BH accretion history in the Universe?

molecular CO emission from z6.42 quasar

• Relation of BH growth and galaxy evolution

• Probing the cosmic reionization

Evolution of Quasar Density
Detection of Gunn-Peterson Trough
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The end of dark ages Movie
Courtesy of N. Gnedin
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How to find the earliest and most distant quasars?

• They are extremely rare
• One per 500 sq. deg at z6 (M
• Require the largest survey of the sky to catch
  them
  
• Search for red, i-dropout objects in the Sloan
  Digital Sky Survey
  
• They are faint at high-redshift
• Require deep follow-up spectroscopic
  observations
  
• SDSS i-dropout survey
• Candidate selection from SDSS
• Fellow-up observations mainly on four work-horse
  telescopes APO 3.5m KPNO 4-m MMT Keck
  

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The Highest Redshift Quasars and Galaxies

• SDSS i-dropout Survey
• Completed in June 2006 7600 deg2 at zAB
• Twenty-five luminous quasars at z5.7
• zmax6.42
• Cosmic age 800 Myr
• The first 6-7 of cosmic history
• Dropout and Ly? emission galaxies
• zspec
• zphot 7 - 8
• GRBs
• 050904 z6.30

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Massive black holes in early universe

• From SDSS i-dropout survey
• Density declines by a factor of 40 from between
  z2.5 and z6
  
• Cosmological implication
• MBH109-10 Msun
• Mhalo 1012-13 Msun
• rare, 5-6 sigma peaks at z6 (density of 1 per
  Gpc3)
  
• Assembly of massive dark matter halo
  environment?
  
• Assembly of supermassive BHs?

Fan et al. 2004
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How fast can a black hole grow?

• Quasars shine by converting potential energy to
  radiative energy when accreting gas
  
• Radiative efficiency of 10
• Quasar maximum accretion rate is limited by the
  presence of radiation pressure (Eddington limit)
  
• At maximum accretion, e-folding timescale of
  quasar growth is 40 million years
  
• Earliest quasars likely grew from seed black
  holes resulted from stellar collapse
  
• Seed mass 10 - 100 M_sun
• To grow a billion solar mass BH needs about 20
  e-folding time - 800 million years, non-stop
  
• The age of the universe at z6 is 800 million
  years
  
• Barely enough time for quasars to grow, even
  non-stop from the big bang???
  

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Surprise 1

• How did black holes grow so quickly in the first
  billion years of the cosmic history?
  
• New (astro)physical processes?
• Direct formation of intermediate mass BH?
• Much more efficient accretion?
• How are the earliest quasars related to the
  earliest galaxies?

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The Lack of Evolution in Quasar Intrinsic
Spectral Properties

Ly a
NV
Ly a forest
OI
SiIV

• Rapid chemical enrichment in quasar vicinity
• High-z quasars and their environments mature
  early on

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Submm and CO observation of z6.42
quasarCo-formation of earliest BH and galaxies

• Strong submm source
• Dust T 50K
• Dust mass 7x108 Msun
• Star-formation rate of 2000 M/yr
• Strong CO source
• Tkin 100K
• Gas mass 2x1010 Msun
• gas, dust properties similar to those of the
  brightest local starburst galaxies

Bertoldi et al.
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High-resolution CO Observation of z6.42 Quasar
VLA CO 32 map

• Spatial Distribution
• Radius 2 kpc
• Two peaks separated by 1.7 kpc
• Velocity Distribution
• CO line width of 280 km/s
• Dynamical mass within central 2 kpc 1010
  M_sun
  
• Total bulge mass 1011 M_sun
• Small, star-forming galaxy hosted over-sized BH
• BH formed before
• complete galaxy assembly?

1 kpc
Walter et al. 2004
Channel Maps
? 60 km/s ?
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Lineless quasars radio quiet BL Lac or quasars
with no BLR?

• No emission line, radio-quiet quasars at z4
• 1 of high-z quasars
• No obvious low-z counterparts
• No BL Lac signature
• A separate population of quasars?

Ly ? distribution
Lineless Quasars EW(Ly?)
Log EW (Ly ?)
Diamond-Stanic et al. 2006
Fan et al. 2006
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Surprise II

• The spectra of these earliest quasars look almost
  identical to those in the local universe
  
• No evolution in spectral properties?
• Mature quasars in a very young universe?
• Black holes grew earlier in the universe?

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reionization
Gunn-Peterson (1965) effect deep HI absorptio
n in high-z quasar spectrum prior to the end of
reionization
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First detection of Gunn-Peterson Effect

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The Universe transforming from opaque to
transparent at the end of cosmic dark ages
transparent
opaque
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Implications of Complete Gunn-Peterson Trough

• G-P optical depth at z6
• Small neutral fraction needed for complete G-P
  trough
  
• By itself not indication that the object is
  beyond the reionization epoch
  
• The evolution of G-P optical depth
• Tracking the evolution of UV background and
  neutral fraction of the IGM
  
• Probe the ending of reionization

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The End of Reionization

• Optical depth evolution accelerated
• z
• z5.7 ? (1z)11

(1z)11
(1z)4.5
Neutral fraction

• Evolution of Ionization State
• Neutral fraction increases by 15
• Mean-free-path of UV photons decreases by 10
• Large variation in the IGM properties
• ? z6 marks the end of cosmic reionization

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Three stages
Pre-overlap
Overlap
Post-overlap
From Haiman Loeb
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Whats Next

• Faint quasar survey at z6
• In deep SDSS stripe
• Additional 10 - 30 quasars at 1-2 mag fainter
• Uses the upgraded MMT red channel - new
  red-sensitive deep depletion CCD
  
• Measures quasar luminosity function at z6
• Probes the inhomogeneity of reionization by
  multiple line of sight
  
• Future IR-based quasars surveys
• On UKIRT, VISTA
• Allows detection at z8-9
• JWST
• Probing the first light at z10

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Probing Reionization History

WMAP