The Highest-Redshift Quasars andthe End of Cosmic Dark Ages

Xiaohui Fan

Collaborators:Strauss,Schneider,Richards, Hennawi,Gunn,Becker,White,Rix,Pentericci, Walter, Carilli,Cox,Bertoldi,Omont,Brandt, Vestergaard, Jiang, Diamond-Stanic, et al. SDSS collaboration

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 upDark --> lightNeutral --> ionized (reionization)

todayCourtesy: G. Djorgovski

Why Distant Quasars? – Existence of supermassive

black holes (BHs) at the end of cosmic dark ages

– BH accretion history in the Universe?

– Relation of BH growth and galaxy evolution

Evolution of Quasar Density

molecular CO emission from z=6.42 quasar

Detection of Gunn-Peterson Trough

– Probing the cosmic reionization

How to find the earliest and most distant

quasars?

• They are extremely rare– One per 500 sq. deg at z>6 (M<-27)

– 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

The Highest Redshift Quasars and Galaxies

• SDSS i-dropout Survey:– Completed in June 2006:

7600 deg2 at zAB<20 – Twenty-five luminous

quasars at z>5.7 – zmax=6.42– Cosmic age ~ 800 Myr– The first 6-7% of cosmic

history

• Dropout and Ly emission galaxies– zspec < 6.6– zphot ~ 7 - 8

• GRBs– 050904 z=6.30

Massive black holes in early universe

• From SDSS i-dropout survey– Density declines by a factor

of ~40 from between z~2.5 and z~6

• Cosmological implication– MBH~109-10 Msun

– Mhalo ~ 1012-13 Msun

– rare, 5-6 sigma peaks at z~6 (density of 1 per Gpc3)

• Assembly of massive dark matter halo environment?

• Assembly of supermassive BHs? Fan et al. 2004

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 z~6 is ~800 million years– Barely enough time for quasars to grow, even non-stop from the

big bang???

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?

NV

OI SiIV

Ly a

Ly a forest

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

early on

The Lack of Evolution in Quasar Intrinsic Spectral Properties

Submm and CO observation of z=6.42 quasar:

Co-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.

High-resolution CO Observation of z=6.42

Quasar• 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< M-sigma predictionSmall, star-forming galaxy hosted over-sized BH

• BH formed before complete galaxy assembly?

Walter et al. 2004

1 kpc

VLA CO 3—2 map

60 km/s

Channel Maps

Lineless quasars: radio quiet BL Lac or quasars with no BLR?

• No emission line, radio-quiet quasars at z>4– ~1% of high-z quasars– No obvious low-z counterparts– No BL Lac signature– A separate population of quasars?

Fan et al. 2006

Ly distribution

Diamond-Stanic et al. 2006

Lineless Quasars:EW(Ly)<10

Log EW (Ly )

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?

reionization

Gunn-Peterson (1965) effect

deep HI absorption in high-z quasar spectrum prior to the end of reionization

First detection of Gunn-Peterson Effect

The Universe transforming from opaque to transparent at the end of cosmic dark ages

transparent

opaque

The End of Reionization

• Optical depth evolution accelerated– z<5.7: ~ (1+z)4.5

– z>5.7: ~ (1+z)>11

(1+z)4.5

(1+z)11

• Evolution of Ionization State:• Neutral fraction increases by >15• Mean-free-path of UV photons decreases by >10• Large variation in the IGM properties z~6 marks the end of cosmic reionization

Neutral fraction

Three stages

Pre-overlap

Overlap

Post-overlap

From Haiman & Loeb

What’s Next

• Faint quasar survey at z~6:– 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 z~6

– Probes the inhomogeneity of reionization by multiple line of sight

• Future IR-based quasars surveys:– On UKIRT, VISTA

– Allows detection at z~8-9

• JWST:– Probing the first light at z>10

Probing Reionization History

WMAP