1. Observations of Obscured Black Holes
Yoshihiro Ueda
(Department of Astronomy, Kyoto University)

2. Search for obscured black holes
Nearly every present-day galaxy contains a BH in its centre with a mass proportional to the spheroid mass, indicating a tight link between the BH and star formation: SMBH is a key ingredient of the universe
Most AGNs are “obscured” (cannot always be distinguished or recognized in other wavelengths). Hard X-ray observations are the most straightforward approach to detect this population without selection biases
In fact, massive star forming galaxies contain rapidly growing BHs heavily obscured by dust (submilimeter galaxies at z~2, Alexander et al. 2005; ULIRGs at z~0, Imanishi et al. 2006). This is consistent with the “co-evoluton” scenario.

3. Star forming history vs
Co-evolution of galaxy and super massive black holes in galactic centres
BH mass vs
Stellar
Star forming history vs
accretion history
e.g., Marconi & Hunt 03
Marconi+ 04

4. The X-Ray Background (XRB)
The XRB is the integrated eission from all the AGNs in the universe, telling us the formation history of supermassive black holes.
The energy density peaks at ~ 30 keV
The shape of the XRB indicates that most of the AGNs are obscured (such as Seyfert 2; Awaki et al. 1993)
The 2-10 keV band is much better than keV, but keV is the best energy band to detect obscured AGNs including Compton thick AGNs
Comastri+ 95

5. X-ray Spectra of Heavily Obscured AGNs
Compton thick AGNs: NH>1024 cm-2 show complex spectra as a function of column density
Reflection/scattered component can be detected below 10 keV but only limited information can be drawn (e.g.,no intrinsic luminosity)
Wilman & Fabian (1999)
Done+ (2003)
NGC 4945
Log NH=24.25
Log NH=24.75
Log NH=25.25

6. What are known from X-ray surveys below 10 keV
Subaru-XMM Deep Survey fields
Log N log S relations (2-10 keV)
1 deg
Kushino+ 02

7. Sample: 1371 AGNs survey N flux limit reference of optical ID
HEAO x Piccinotti+82, Grossan+82
ASCA x Akiyama+00, Akiyama+03, Ishisaki+01
Chandra x Barger+03, Szokoly+04, Zheng+ 04
ROSAT/XMM/Chandra x Hasinger+ 05 and therin

8. (1)+(2) → Population Synthesis Model
The current status of X-ray surveys
The XRB below ~ 6 keV has been almost completely resolved and identified and hence is well understood.
Howerver, even with the deepest Chandra/XMM surveys a significant fraction of the XRB remains unresolved above 6 keV (Worsley et al. 2004). Above 10 keV only 1 percent of the XRB is resolved at present.
Extrapolation has to be made beyond the observational results.
Population synthesis model reproducing the XRB spectrum
Given the luminosity function and absorption function determined below 10 keV, we predict contribution of Compton-thin AGNs to the background above 10 keV with an assumption of a broad band spectrum over keV
The missing background is then be attributed to Compton thick AGNs

9. Population synthesis model
The observed XRB spectrum
Integrated AGN emission from our HXLF and the absorption function (lower black) well reproduces the broad band XRB spectrum (blue) below 300 keV
High energy cutoff must be arround 500 keV in average
(red left: keV,
red right: 600 keV)
Presense of Compton-thick AGNs estimated by Risaliti et al (1999) is consistent with the XRB spectrum (upper black)
Reflection components are important
(green: no reflection)
Ueda+ 2003

10. Compton thick AGNs or Compton reflection?
The fraction of Compton thick AGNs, introduced to reproduce the intensity XRB spectrum at 30 keV, is coupled with the amount of reflection component (assumed to be Ω=2πfor both type-1 and type-2 AGNs)
Precise study of broad band spectra of neaby AGNs (especially type-2 AGNs) is crucial. Suzaku observations are important.
Integrated spectrum
of type-1 AGNs
Compton-thick AGNs
(keV)
Observed XRB spectrum
Ueda+ 2003

11. A big remaining issue: The number density of Compton thick AGNs
Significant contribution to the SMBH mass growth?
“AGN relic” black hole mass function can be calculated from the luminosity function of the “whole AGNs” including Compton thick ones
For instance, Marconi et al. (2004) have to assume 0.6 times additional Compton thick AGNs as many as Compton thin ones to reproduce the local BH mass function
In the local universe the number density of Compton thick AGNs may be comparable to or even larger than Compton thin AGNs (Maiolino et al. 2003)
At higher redshifts, little is known about the number density of Compton thick AGNs
Mid IR + radio selection of type-2 QSOs at z~2 implys twice as large number density as Compton thin type-2 QSOs (Martines-Sansigre et al. 2006)

12. Evidence for Compton thick AGNs in the local universe
A population of infrared galaxies that do not show AGN signatures in their optical spectra are found to be Compton thick AGNs by Chandra follow-up (optically elusive AGNs)
The number density is comparable to that of Seyfert 2 galaxies
Their nucleus is completely hidden so that no narrow-line region form?
Swift/BAT or INTEGRAL surveys will give a definete answer for this at least for those with log 24 <NH<25
Maiolino et al. (2003)

13. log N log S relation above 10 keV
If we simply extrapolate the NH function below log NH=24 to log NH=26 based on the Ueda 2003 model, then
The fraction of Compton thick AGNs is predicted to be ~10% (Fx=1e-11) to ~25% (Fx=1e-16)
NeXT limit
~40-50% XRB
2-8 keV Survey
10-30 keV Survey
Fraction of Compton thick AGNs
Ueda+ 03

14. Summary
There is growing evidence for the presence of a large population of Compton thick AGNs in the universe.
We have not fully understand the XRB origin yet. The population synthesis model is being almost established below 6-8 keV. However, we have to note that there are a few critical assumptions are made when extrapolating it to above 8 keV.
To fully understand the accretion history of the universe, it is critical to reveal the evolution of Compton thick AGNs.
Sensitive hard X-ray surveys above 10 keV with various depths and widths, as done at energies below 10 keV, are the only way to unveil this problem.