1. The Most Distant Quasars
Xiaohui Fan
University of Arizona
June 7, 2010
Collaborators: Brandt, Carilli, de Rosa, Jiang, Kurk,
Richards, Schneider, Shen, Strauss, Vestergaard,
Walter, Wang
Today I would like to present some new Spitzer observations of the hot dust emission from dust tori in the
Mosts distant quasars at z~6. The question that we would like to ask is really:
Background: 46,420 Quasars from the SDSS Data Release Three

2. Quasar of the day
Last night’s astro-ph: Willott et al. new highest-redshift quasar at z=6.44

3. Quest to the Highest Redshift

4. 30 at z>6 60 at z>5.5 >100 at z>5
Here are the spectra of some 30 quasars at z~6 range, most discovered in the SDSS over the last eight years or so.
We notice two features, one is the strong Ly alpha absorption, the presence of complete G-P absorption trough at z>6
Indicating rapid increase in the neutral H density in the IGM as we are approaching the reionization epoch.
The other is again the existence of strong metal lines, even at the highest redshift, lines from C, N, O, Si, all the way to Mg, and Fe.

5. Key Questions When did the first supermassive BH form?
Measurement of quasar luminosity function and BH mass at z>6
When did the first quasar form?
(lack of ?) Evolution of spectral energy distribution
Co-evolution of the earliest BHs and galaxies
Does M-σ relation exist at z>6?

6. Formation of z~6 quasars from hierarchical mergers
Li et al. 2007

7. Theorists Tell us Li et al. 2007 These luminous z~6 quasars:
The most massive system in early Universe
Living in the densest environment
BH accreting at Eddington
Host galaxies have ULIRG properties with maximum starburst
Li et al. 2007

8. Quasar Evolution at z~6 Strong density evolution
Density declines by a factor of ~40 from between z~2.5 and z~6
Black hole mass measurements
MBH~ Msun
Mhalo ~ Msun
rare, 5-6 sigma peaks at z~6 (density of 1 per Gpc3)
Luminosity function at z~6
Bright end slope steep
LF breaks at M~-25
Not likely significant contributor to reionization budget
bad news for deep quasar surveys
Fan et al. 2006
Note: 1. They are extremes. Sensitive tests; 2. But also rare object, so don’t generalize too much
Low-z
z~6
Willott et al. 2010

9. Eddington Ratios in z~6 Quasars
Quasar BH mass measured from near-IR spectroscopy in CIV and MgII regions
On average: at or close to Eddington accretion
z~6 quasars
See De Rosa poster

10. Are there luminous quasars at z>>7
Black Holes do not grow arbitrarily fast
Accretion onto BHs dicitated by Eddington Limit
E-folding time of maximum supermassive BH growth: 40 Myr
At z=7: age of the universe: 800 Myr = maximum 20 e-folding
Billion solar mass BH at z>7
Non-stop, maximum accretion from 100 solar mass BHs at z=15 (collapse of first stars in the Universe)
Theoretically difficult for formation of z>7 billion solar mass BHs by Eddington-limited accretion from stellar seeds
What if we find them:
Direct collapse of “intermediate” mass BHs?
More efficient accretion model “super-Eddington”?
So where are those earliest quasar? Is it because the universe simply can not make them fast enough? Jury is still out in term of what the constraint on the density of these objects are. But there is an important theoretical consideration that people are taking very seriously now, and that’s the concept of Eddington limit, which in principle is the fastest rate a BH can grow in a steady state, because in a steady state, a particle far away from the BH must at least by balanced by radiation pressure from the BH, otherwise things will be blown apart, that means for a given mass, there is a max luminosity, called Edd luminosity, and since BH grows by converting gravitational potential energy to radiation, luminosity is propotional to the growth rate, M dot, therefore this simply differential equation essentially means that in normal circumstances, a BH can e-fold its mass in 40Myr, at the maximum. Comparing to the age of the universe, 40Myr is a short time, but at high redshift, this is becoming an interesting fraction of the age of the universe, in other words, the BH in early universe might be bound by Edd limit and limited by the number of e-folding available.

11. non-evolution of quasar (black hole) emission
z~6 composite
Ly a
Low-z composite NV
Ly a forest OI
SiIV
XF et al. 2010
Jiang, XF et al. 2008
In some sense, we were quite surprised to see this general lack of evolution in the quasar spectral properties.
It is best illustrated here, where we compare the composite spectrum of our sample of z~6 quasars with the composite from all SDSS quasars, at average redshift of 1.5 or so. It is striking that in terms of both emission line strength and continuum shape, there is no evolution whatsoever from the local universe to the highest redshift currently observed, close to reionization epoch, The blue side of Ly alpha emission of course is affected by IGM absorption.
People are still debating exactly how significant this is; but the bottom line is that these quasar evnv has reached high metallicity early on with no or little evolution, and the high-z quasars we observe, in term of the hot gas around them, are old, mature.
Rapid chemical enrichment in quasar vicinity
Quasar env has supersolar metallicity : no metallicity evolution
High-z quasars are old, not yet first quasars, and live in metally enriched env similar to centers of massive galaxies

12. When did the first quasar form?
Dust: emitting in infrared
To probe this, we have been observing these quasars not only in the optical wavelength we found them, but for photons of all energies, from X-ray to radio, because different wavelength photons come from different part of the region surrounding central BH in quasar, roughly speaking, high energy photons comes from the most inner part, only light hours to light days from BH, and probably can form very quickly, and low energy photons, such as those emitting in the IR, comes from extended dust structure light years away. And these dust emission in IR is what the next discovery came from.
radiation from X-ray to radio as a result of black hole accretion and growth

13. Hot dust in z~6 Quasars
Lack of evolution in UV, emission line and X-ray  disk and emission line regions form in very short time scale
But how about dust? Timescale problem: running out of time for AGB dust
Spitzer observations of z~6 quasars: probing hot dust in dust torus (T~1000K)
Three unusual SEDs among ~30 objects observed.
dust
No hot dust??
Jiang, XF et al. 2006, 2010

14. Disappearance of Dust Torus at z~6?
typical
J0005
3.5m
4.8m
5.6m
8.0m
16m
24m
quasars with no hot dust
Spitzer SEDs consistent with disk continuum only
No similar objects known at low-z
no enough time to form hot dust tori? Or formed in metal-free environment?
In cycle 4, we found another object, with non-detection in the mid-IR, and SED consistent with disk emission only, as if there is no dust torus at all. As shown in these Spitzer images, a typical quasar has strong mid-IR emission, but these quasars simply dropped out from mid-IR. The crucial point here is that no object with such weak IR emission has ever been detected, at any redshift, before. As shown in this diagram, which plots the host contribution, in term of hot dust flux vs. optical flux, for all Spitzer objects from z=0 to 6, and our IR weak quasars stands out as the only example, and they happen to be some of the most distant object. Is this a coincidence? Or is this showing something fundamental about quasar dust properties? Of course not all quasars at z~6 have weak hot dust emission, so what’s so special about them?
Jiang, XF et al

15. Epoch of first quasars? Dust-free quasars: Dust/Bolometric
Only at the highest redshift
With the smallest BH mass
First generation supermassive BHs from metal-free environment?
How are they related to PopIII?
Dust/Bolometric
Dust/Bolometric
Jiang, XF et al. 2010
BH mass

16. Probing quasar host galaxies at high-z
[OIII]
Direct imaging: hard!
Quasar host galaxies very hard.
Two ways out: type-2 quasars? How to get BH mass?
Low Eddington ratio quasars - they are not that many.
Most hopeful is probably CO..
Radio/sub-mm! CO

17. Star Formation in z~6 Quasars
30% of z~6 quasars detected at 1mJy level in 1-mm ->
LFIR~ 1013Lsun
T~50K
SFR~1000 Msunyr-1 (if dust heated by SB)
New CO observations
eight quasars detected in CO
Probing ISM properties and host galaxy masses
Wang et al. 2008, 2009

18. Maximum starburst in z=6.4 quasar ?
Spatially resolved CO and [CII] emissions:
Size: ~1.5 kpc from [CII] (0.3”)
Continuum has >50% extended component: SB heating?
Star formation rate of: ~1000 Msunyr-1kpc-2
Eddington limited maximum star formation rate (Thompson et al.)?
Gas supply exhaused over a few tdyn
Similar SF intensity to Arp 200 but 100 times larger!
Dynamical mass:
CO/CII line width ~300km/s
Dynamical mass ~1011Msun?
BH formed earlier than completion of galaxy assembly?
1kpc
Walter et al. 2004
Walter et al. 2009

19. Do z~6 Quasars Live in the Densest Environments?
High-redshift quasars are strongly clustered
But efforts to look for overdensity around z~6 quasars have mostly produced non-results (Willott et al., Kim et al., Kurk et al., Zheng et al.)
Shen et al. 2007

20. Do z~6 Quasars Live in the Densest Environments?
Non-detection of significant overdensity around z~6 quasars:
Quasars suppress dwarf galaxy formation?
Quasar hosts are not massive?
Needs deeper and wider surveys
Overzier et al. 2008

21. Conclusions and Questions
Rapid evolution of quasar density at z~6
Are we closing in to the epoch of the earliest SBH formation?
First hot dust at z~6
Are we closing in to the epoch of first AGN structure?
Luminous quasars seem to live in modest environments
Narrow CO line width  small host mass
No significant overdensity of galaxies
How closely tied are the earliest SBHs and galaxies? Or are we just picking up early starters in term of BH accretion in the most luminous quasars?
Important changes at z~6: needs to push for higher redshift and lower luminosities

22. Quest to the Highest Redshift

23. Quest to the Highest Redshift
090423
080913
050904
000131
GRBs
970228

24. Probing Reionization History
WMAP
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.