Probing the End of Reionization withHigh-redshift Quasars

Xiaohui Fan

University of Arizona

Mar 18, 2005, Shanghai

Collaborators: Becker, Gunn, Lupton, Narayanan, Penterrici, Richards, Rix, Strauss, White et al.

And SDSS collaboration

Courtesy of Arizona graduate students

The Highest Redshift Quasars Today

• z>4: >900 known • z>5: >50 • z>6: 8• SDSS i-dropout Survey:

– By Fall 2004: 6000 deg2 at zAB<20

– Sixteen luminous quasars at z>5.7

– Many L and T brown dwarfs

• ~30 at z~6 expected in the whole survey

High-redshift Quasars and the End of Cosmic Dark Ages

– Existence of SBHs at the end of Cosmic Dark Ages

– BH accretion history in the Universe?

– Relation of BH growth and galaxy evolution?

Evolution of Quasar Density

Resolved CO emission from z=6.42 quasar

Detection of Gunn-Peterson Trough

– Probing the end of reionization?

Searching for Gunn-Peterson Trough

• Gunn and Peterson (1965)– “It is observed that the continuum of the source

continues to the blue of Ly-α ( in quasar 3C9, z=2.01)”

– “only about one part of 5x106 of the total mass at that time could have been in the form of intergalactic neutral hydrogen ”

• Absence of G-P trough the universe is still highly ionized

Keck/ESI 30min exposure

Gunn-Peterson Trough in z=6.28 Quasar

Keck/ESI 10 hour exposure

White et al. 2003

Strong Evolution ofGunn-Peterson Optical Depth

Fan et al. 2004

Transition at z~5.7?

Implications of Complete Gunn-Peterson Trough

• G-P optical depth at z~6:

– 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

)/(10GP

5~ HHI nn

Three stages

Pre-overlap

Overlap

Post-overlap

From Haiman & Loeb

Evolution of Ionizing Background

• Ionizing background estimated by comparing with cosmological simulations of Lyman absorption in a LCDM model– Ionizing background declines

by a factor of >25 from z~3 to z~6

– Indication of a rapid decline at z>5.7…

– Marks the tail end of reionization – overlapping of individual HII regions – at z~6

Fan et al. 2002, 2005

Photoionizing rate

Gunn-Peterson Troughs in the

Highest-redshift Quasars

• Four quasars known at z>6.2• Strong, deep Lyα and Lyβ

absorption in all four objects immediately blueward of Lyα emission…

• None of the quasars at z<6.2 shows any G-P troughs– There is no Gunn-Peterson

trough at zabs<5.8– Transition happens at

z>5.8…• But LOS variation is

significant…

Line of Sight Difference…

White et al .2003

universe highlyionized in this line of sight!

Completely darkG-P trough

Leaky IGM at z~6

• Deep narrow band ACS imaging – Lyβ transmission point-like,

coincides with quasar position

IGM transmission at z~6, not intervening galaxies, not lensed…

• Gunn-Peterson Ly γ trough

τ < 15, line of sight highly ionized (Oh & Furlanetto 2004)

White, Becker, Fan and Strauss 2004

Line of sight variations at the end of overlapping?

– The “last transmitting” redshift ranges from 5.85 to 6.15

– Non-uniform reionization?– Expected if ionizing sources rare/clustered and HII

bubbles have large sizes?

Size of ionized bubbles at the end of reionization• Wyithe and Loeb (2004)

– Completion of overlapping determined by the light crossing time of HII bubbles larger than the scattering of bubble formation time

Scatter in the overlapping redshift of the order Δz ~ 0.13, with a bubble size of 60 co-moving Mpc at z~6

– Consistent with scatter seen in the last transmission redshift of z~6 quasars

Constraining the Reionization Epoch

• Neutral hydrogen fraction– Volume-averaged HI fraction >

0.1% at z~6

• From G-P alone:– There is still a long way to go

from τ>10 to τ~100,000

– Gunn-Peterson test only sensitive to small neutral fraction and saturates at large neutral fraction

– Need more sensitive tests to probe larger neutral fraction

Fan et al. in prep

mass ave.

vol. ave

Size of Stromgren Sphere

• Sharp transition at the end of quasar proximity zone implies the boundary of HII region around quasar

• Size of HII region a function of quasar life-time and neutral fraction of the IGM– Assume 107 yr lifetime n(HI)

higher than 10% at least for some line of sight

– Calculation dependent on lifetime as well as detailed radiative transfer…

Wyithe and Loeb, Mesinger and Haiman

Put the picture together• G-P results: overlapping might be

happening at z~6 or slightly earlier

• CMB polarization shows: substantial ionization by z~17

• These two measurements are not contradictory; combining GP with CMB complex reionization history:– First star formation happens very

early?

– Reionization last from 20 to 6? (600 million years)

– Reionization is not a phase transition

– Multiple episodes of reionization?

?

Probing the Reionization Epochbefore JWST

• More lines of sight from high-redshift quasars– About 20 – 40 quasars 6 < z < 6.6 from SDSS

– Spectroscopy in far-optical and near-IR with large telescopes

• G-P test: probing non-uniformity of the end of reionization

• Metal absorption lines at z>6

Probing the first Metals in the IGM?

5 – 10 hour exposure in the far-optical and near-IR of z~6 quasars on 6 – 10m class telescope

High S/N to detect intergalactic CIV and SiIV absorptionTo probe the metal density of the IGM at z>5

10 hour Keck/ESI

J1148+5251

Evolution of IGM CIV density

• No redshift evolution of CIV density from z~2 to 5

• IGM enriched in metal at z>>5

• Future observations:– Near IR spectroscopy: metals at

z~6

– How does the IGM enrichment history trace the reionization history?

– Provide more sensitive probe to the neutral fraction?

Pettini et al. 2003

?

Probing the Reionization Epochbefore JWST

• More lines of sight from high-redshift quasars– About 20 – 40 quasars 6 < z < 6.6 from SDSS

– Spectroscopy in far-optical and near-IR with large telescopes

• G-P test: probing non-uniformity of the end of reionization

• Metal absorption lines at z>6

• Looking for objects at z>7: first sources of light– z~6.6 is the limit of optical survey IR searches, UKIDSS, VISTA

etc.

– High-redshift GRBs: SWIFT!!!

• 21cm mapping of reionization

mapping the complete history of reionization and early galaxy formation