# Presentation: 815358

##  Observation Constraints of the End of Reionization

** ****Observation Constraints of ****the End of Reionization**

***Xiaohui Fan***
***University of Arizona***

- Collaborators: Becker, Carilli, Ferrara, **Gallerani**, **Jiang**,
- Richards, Roy Choudhury, Strauss, **Xu**, Walter, White, et al.

- Background: 46,420 Quasars from the SDSS Data Release Three

##    The Highest Redshift Quasars Today

- z>4: >1000 known
- z>6: 19
- SDSS i-dropout Survey:
- Completed in June 2006:
***7700 deg******2,  ******z******AB****** < 20***
***27 luminous quasars at 5.71<z<6.42***
***CFHT High-z Quasar Survey (CFHTQS, Willott et al. astro-ph/0706091)***
***Goal: 400 deg******2******, z******AB******<22.5***
***4 quasars at z>6***
***New highest-z quasar at z=6.43***
***SDSS Faint Quasar Survey (SFQS)****: *
**faint quasars in the deep SDSS stripe (****Jiang, XF et al.****),  **
**300 deg****2****, z****AB**** < 22.5**
**six z~6 quasar at 20 < z****AB**** < 21**
**Goal: quasar LF**

- Other on-going z~6 quasar surveys:
***AGES**** (Cool et al.): **Spitzer** **selected**, one quasar at z=5.8*
***FIRST-Bootes**** (Becker et al.): radio selected, one quasar at z=6.1*
***QUEST:**** i-dropout surveys similar to SDSS*
*IR-based survey: ****UKIDSS, (z=5.83), VISTA****,  allows detection up to z~8-9.*

**Notes:**

We have been carrying out a survey for high-z quasars using SDSS for the past five-six years.
 Now we have surveyed more than 6000 sq.deg. 
The high-z quasar sample now includes about 1000 quasars at z>4, 
allows us to beat down statistics and study the evolution of population. 
So far we have discovered 19 quasrs at z>5.7. 
The highest is still the 6.42 quasar discovered three years ago. 
The sdss imaging data collection will come to a close in the next couple of seasons. 
We are now using the coadded data in areas with multiple observations 
to push for fainter mag, and hopefully will be able to increase this sample. 

## Slide 3

**Notes:**
Here are the spectra of the 19 quasars, from 5.74 to 6.42. Note the two 
prominent spectral features: the presence of strong emission lines, 
not only ly a, but also strong metal lines, from N, C, Si etc., indicating
 high metallicity in the quasar env; 
and the strong evolution and complete absorption of ly a forest, 
where the complete or almost complete g-p troughs appear at z>6. 
Now we will discuss the properties in detail. 

## Quasar Density at z~6

- From SDSS i-dropout survey
- Density declines by a factor of ~40 from between z~2.5 and z~6
- Cosmological implication
***M******BH******~10******9-10******  M******sun***
- Mhalo ~ 1012-13 Msun
- rare, 5-6 sigma peaks at z~6 (density of 1 per Gpc3)
***Assembly of supermassive BHs?***
***The universe is only ~20 t******eddington****** old, requiring non-stop Eddington accretion of 100 M******sun****** from z>>20***
- Is Eddington-limited accrection from stellar seeds still permitted, or are alternative accretion modes (super-Eddington, intermediate mass BH) needed?

- Fan et al. 2006

** **

- NV

- OI

- SiIV

- Ly a

- Ly a forest

- Rapid chemical enrichment in quasar vicinity
- Quasar env has supersolar metallicity : no metallicity evolution
- High-z quasars are ***old, *****not yet first quasars..**

***The Lack of Evolution in Quasar Emission Line Properties***

- Fan et al.2007

- z~6 composite

- Low-z composite

**Notes:**
The other major findings of evolution of the highest z quasars is the striking lack of evolution
 in the spectral energy distribution of quasars, which is in sharp contrast to the strong 
 density evolution, and is highlighted here:
The composite spectrum of z~6 quasars is almost identical to z~0 quasars
 in terms of both slope and emission line strength.
This similarity also extends to other wavelengths, in terms of X-ray, IR and radio prop.
The lack of SED evolution indicates that quasar environment have high metallicity
  and rapid enrichment, and quasar evn matured very early on. 

## Quasar Metallicity at z~6

- Jiang, XF et al. 2007

- near-IR spectroscopy:
- Gemini + Keck

## Two Key Constraints:

- From Avi Loeb

**reionization**

- WMAP 3-yr: zreion=10+/-3
- 2.  IGM transmission: zreion > 6

## End of Reionization Epoch:Open Questions

- What’s the Status of IGM at z~6?
- Measurements of Gunn-Peterson optical depth
- Evolution of UV background
- Constraints on IGM neutral fraction
- Was the Universe mostly neutral by z~6-8?
- Distribution of dark gaps
- HII region size distribution
- Evolution of  Lyman alpha emitters
- What is the source of reionization?
- First galaxies or AGNs

## 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 at z~6***

## Gunn-Peterson Trough

***Keck/ESI 30min exposure *********

**Gunn-Peterson Trough**
** ****in z=6.28 Quasar**

***Keck/ESI 10 hour exposure *********

- White et al. 2003

## Evolution of Lyman Absorptions at z=5-6

******z = 0.15**

**Notes:**
OK, now switch gear and talk about IGM.
We all know the strong evolution of lyman absorption at high-z.
Now I will show two animations: they are all segments of observed ly alpha or ly beta
 transmission, with z range of 0.15, or about 60 comoving Mpc at z~6
I will show these independent transmission spectrum from low to high-z.
The full scale is 0 - 0.4. They all have same spectral resolution.
At z~5 to 5.5: 
On average the absorption becomes stronger
But there are still few completely dark pixels.
Most regions have some transmission
Although occasional long dark gaps present at z~5.3
A sharp transition happened at z~5.7
From there, forest is dominated by long, dark gaps, complete GP
With only occasional transmitting pixels. 
Show again, two important points
The absorption strength evolution accelerated at z~5.7 - 5.8
The appearance changed from most pixels transmitting to most pixels opaque
But with very significant line of sight variance
And even at z>6, there are still transmitting pixels, I.e., the GP trough is not inifinitely long.
Now ly b:
 the available sight line is shorter due to contamination of high-order foreground abs
But the evolution and transition at z~5.7 is sharper. More pronoucd.
Due to the smaller o.s. and higher sensitivity of ly b.
Now let’s look at the IGM evolution more quantitatively. 

## Accelerated Evolution at z>5.7

**Accelerated Evolution at z>5.7**

***Optical depth evolution accelerated***
- z<5.7:  ~ (1+z)4.5
- z>5.7:  ~ (1+z)>11
***End of reionization?***
***Dispersion of optical depth also increased***
- Some line of sight have dark troughs as early as z~5.7
- But detectable flux in ~50% case at z>6
***End of reionization is not uniform, but with large scatter***

- (1+z)4.5

- (1+z)11

- XF et al. 2006

**Notes:**
Now let’s combine those two. We are trying to be careful in converting alpha and
 beta optical depth. We use both a photoionization model in a clumpy IGM, similar to
  what we did previously, and use a n empirical calibration, by requiring ly and ly b give
  consistent average effective transmission at the same redshift, to calculate the conversion.
The conversion factor, because of the clumpyness of the IGM, which allows flux
 to come through in underensed regions, is about 2.5, a factor of 2.5 smaller than the factor of 
 6.2 for the uniform case.
Looking at the green ly beta and blue ly a points, they give consistent measurements when
  there are detections in both transitions.
  when there is no ly a transition, ly beta sometimes give clear detections that are consistent
  with ly a upper limit, some times give a much more stringent upper limit when no flux is detected
   even in beta transition
Combining the two, we see clearly that:
 1. The evolution of optical depth accelerated. At z>5.7, the best fit power law has a index
  (1+z) to 10 or larger, although there are still about 50% case that there are clear detections,
  I.e., the optical depth is finite.
 2. Shaded area is 1-sigma dispersion of tau.
  The dispersion of optical depths also increased dramatically.  At z<5.9, the relative scattering
 is 0.3 - 0.5, while at z>6, the dispersion is of the order of unity, while some dark regions
 begins to appear as early as 5.7 or so. 

## Evolution of Ionization State

- UV Ionizing background:
- Assuming photoionization and model of IGM density distribution
- UV background declines by close to an order of magnitude from z~5 to 6.2
***Increased dispersion suggests a highly non-uniform UV background at z>5.8***

- From GP optical depth measurement, volume averaged neutral fraction increase by >~ order of magnitude from z~5.5 to 6.2

- XF et al. 2006

- Neutral fraction

- UV background

**Notes:**
Using the optical depth measurements, we can estimate the UV background level,
  assuming photoionization in a clumpy IGM. 
We find that similar to optical depth, the evo of background level, here expressed in term of
Photoionization rate, in the unit of 10^-12 per second, accelerated at z>5.8 or so. 
The average UV background declines by close to an order of mag from 5 to 6.2.
The IGM length that we use for measurement is ~60 Mpc, large enough that the total density
 fluctuation is typically not important. In this case, the increased dispersion of estimated
 UV background has to come from the fluctuation of ionization state. So I think what we are
 seeing here is  a large dispersion as a result of a highly non-uniform UV background, 
 something one should expect to see as we move close to reionization.

## Relation between optical depth and neutral fraction highly model-dependent

**Relation between optical depth and neutral fraction highly model-dependent**

- Becker et al. (2006)
- optical depth evolution could be consistent with a smooth evolution of ionizing background for a **strongly-evolving log-normal density distribution** of the IGM

- Relation between  and  dependent on IGM clumpiness
- Simulation results: clumpiness ~ 5 - 30, no strong evolution at z~6
- Optical depth evolution driven by an decreasing background towards high-z

## Evolution of Proximity Zone Size Around Quasars

**Evolution of Proximity Zone Size Around Quasars**

- Size of Proximity Zone region
-     Rp ~ (LQ tQ / fHI )1/3
- Size of quasar proximity zone  decreases by a factor of ~2.4 between z=5.8 and 6.4  (Fan et al. 2006)
**Consistent with neutral fraction increased by a factor of ~15 over this narrow redshift range**
**Can be applied to higher z and f****HI**** with lower S/N data**
- Actual size of proximity zone dependent on details of radiative transfer and quasar model...

- XF et al. 2006

- Shapiro, Haiman, Mesinger, Wyithe, Loeb et al.

- Proximity zone size (Mpc)

- redshift

**Notes:**
The third way to measure IGM ionization is to look at the HII region size that Zoltan talked about
In the last talk, which seems to give the largest overall estimate previously. 
Here we try to look at all objects. Since  the absolute values of HII region estimate
 depends on a number of parameters, such as quasar lifetime, details of radiative transfer,
 which are not well known, we here concentrate on  the the relative evolution of HII region size
 first, Because if there is some kind of phase transition at z~6 after which the neutral fraction
 is very high, then we should simply see a dramatic decrease in the HII region size  as 
 it is confined by expanding into a largely neutral IGM. 
We don’t have time to talk about details of defining the HII region size, but basically, if one
Uses a consistent definite, one finds that the size of proximity zone around these quasars,
 esp. after correcting for luminosity difference, deceased by a factor of more than 2 from 
 z=5.8 to 6.4. The measurement error is large, so there is only a 3 sigma result.
But it tells us two things:
 first, there is a measurable difference in the HII region size, indicating that the model
 of HII region expanding into an increasingly neutral IGM is reasonable,  and the most
Straightforward explanation is that the neutral fraction increased by, again, order of mag
Over this narrow z range.
Second, however, we don’t see a huge increase yet, in other words, it doesn’t seem to
 a jump of several order of mag which is required if we want to IGM to be mostly neutral.

## Uncertainties in interpretation of proximity zone sizes

- Bolton & Haehnelt (2006), Maselli et al. (2006)
- Observed size of proximity zone much smaller than true HII region size
- Neutral fraction <~ a few percent
- Consistent with G-P constraints
- Mesinger et al. (2004),  Wyithe et al. (2005)
- Neutral fraction ~10-30%
- Better models and simulated spectra needed...

- Maselli et al.
- 2006

- Bolton & Haehelt 2006

## CFHTQS results (Willott et al. 2007)

- Strong evolution in GP seen at z>5.5, but low S/N
- Scaled near-zone size 6-11 Mpc at z>6
- Consistent with a low fHI
- But uncertainties in zem based on Ly
**Caution: using Ly******** redshift could introduce large uncertainty in HII region size; but true systematic redshift difficult for faint quasars at z>**6

## Dark Gap Distributions

- Dark gap statistics (Songaila & Cowie 2002)
- Gaps: regions where all pixels have >2.5
- Gaps among z~6 quasars
- Average length shows the most dramatic increase at z>5.8  IGM is dominated by long, dark gaps
- Consistent with overlap at z~6-8?
- Dispersions
- Even at z>6, gap lengths are still finite
- Upper limit on neutral fraction
- If IGM largely neutral, GP damping wing will wipe out all HII region transmissions
***Existence of transmission at z>6 places an upper limit of average neutral fraction <30% (Gallerani et al. 2007)***
- Independent upper limit on neutral fraction

- XF et al. 2006

**Notes:**
In these two plots, we show the evolution of distribution of dark gaps in a and b transitions.
Here a dark gap is defined as a region where all pixels in the spectrum has optical depth
 larger than 3.5. We are using Keck/ESI spectra with a resolution of 4000 or so, and this
 limit of 3.5 is more or less what the spectra gives you for most of the pixels. 

What we find is that for both alpha and beta, the average length of dark gaps in the spectra
 shows the most dramatic evolution: IGM at z<5.7 is dominated by transmitted pixels with
 only short dark gaps, and very occasionally long gaps, IGM at z>5.8 is dominated by very
 long gaps, in many cases with lower limit. We think this really indicate some fundamental
 changes in the ionization state. Of course this change is not uniform, again with large 
 l.o.s. variation.

The other thing I want to point out is that even at z>6, the gap length is still finite, in other words
 there are still transmitting pixels as we see before. 
Using this fact, we can actually put a robust upper limit on the neutral fraction. 
Because we know that star forming galaxies at z~6 will blow up HII bubbles even if IGM is 
 neutral, but the damping wings from a neutral IGM will suppress and wipe out the transmission
 from early HII bubbles. So the presence of transmitted pixels tells you that IGM can’t be
 totally neutral, which is completely independent from GP optical depth measurment above.
This is very similar to using LF of ly a emitters to constrain neutral fraction., 
We actually found a simialr limit, that the existence of transmission at z>6 places an upper
Limit of neutral fraction to be smaller than 30% or so. Again, it is an independent estimate.

## Dark Gap Statistics: Comparison with Simulations 

- Gallerani et al. (2006, 2007)
- Early reionization (zoverlap ~15)
- Late reionization (zoverlap ~7)
- Significant difference in gap distribution at z~6
- fHI<0.3 at z~6
***But observed transmission peaks too wide compared to simulations***

- Distribution of width of transmission peaks

- Distribution of dark gaps

- 5.7<zem< 6.0

- 6.0 <zem< 6.4

## Ly  Galaxy LF at z>6

- Iye et al. 2006
- Kashikawa et al. 2006

**Ly ******** Galaxy LF at z>6**

- Neutral IGM has extended GP damping wing  attenuates Ly  emission line
- New Subaru results
- Declining density at z~6-7 (2-3 result)
- Reionization not completed by z~6.5
- Neutral fraction could be as high as a few tenths but strongly model-dependent
- cf. Malhotra & Rhoads, Hu et al.: lack of evolution in Ly  galaxy density

## GRBs as Probes of Reionization

- Detected to z=6.30
- Advantages:
- Bright
- Flat K-correction due to time dilation at high-z
- Small surrounding HII regions: could use damping wing of Gunn-Peterson trough to probe high neutral fraction

- Constraining neutral fraction
- How to distinguish internal absorption from IGM damping wing??
- Using 050904: fHI < 0.6 (2-sigma) by fitting both DLA and IGM profiles

**Damping wing?**

- GRB050904

- Kawai et al. 2005

## What Ionized the Universe? AGNs or Galaxies

**Quasar LF at z~6:**
- SDSS Wide: 7700 deg2, 17 quasars, zAB  <20
- SDSS Deep: ~150 deg2, 6 quasars, 20<zAB<21
- AGES: 1 quasar in 5 deg2 at zAB<21.5
**Steeppening of  LF:**
*********L******-3.1***
***Comparing to ************L******-2.4 ******at z~4***

- Jiang, XF et al. 2007

- SFR of galaxies

- Density of quasars

##  Reionization by AGNs?

- Can quasars do it?
- Too few quasars unless QLF remains to be steep to AGN luminosity
- Can low-luminosity AGNs ionize the IGM by z~6?
- Stacking X-ray image of LBGs in UDF... too few faint AGNs
- Can accretion to seed BHs ionize the IGM by z~15?
**Dijkstra, Haiman & Loeb (2004)**
**Soft X-ray background overproduced if quasars produce ~10 photons/H atom**
**‘****Preionization’ to  f(HI)~50% by X-rays is still allowed (e.g. Ricotti et al.)**

- Jiang, XF et al. 2007

## Summary  

- IGM evolution accelerated at z>6
- Neutral fraction increased by order of mag from z=5.5 to z>6
- fHI a few percent, but highly model-dependent
***z~6 marks the end of overlapping stage of an inhomogeneous reionization***
- z~6 quasars are old; lots of BH build-up and chemical enrichment at high-z
- AGN not likely sources of reionization

## Probing Reionization History

***WMAP***

**Notes:**
Finally, what does it mean for the reionzation history?
I think what we are seeing is:
 at z~6, the IGM neutral fraction increased by an order of mag, or more,
 but it is probably not yet neutral at that time.
 if this trend continues, then the IGM could be largely neutral by z of 8. 
 in this case, we might really need a double reionization model to explain everything.,
 so we simply need some z~7 to 8 objects, whether quasar or GRB, to answer these
 questions, if the IGM is neutral, we should detect very long GP troughs in these objects. 

## Surveys of quasars at z~7

- LBT: LBC-Red
- i-z-Y selection (1 deg2/night)

- UKIDSS: YJHK photometry