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Observations of
Obscured Black Holes
Yoshihiro Ueda
(Department of Astronomy, Kyoto University)
Search for obscured black holes
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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 “coevoluton” scenario.
Co-evolution of galaxy and super massive
black holes in galactic centres
BH mass vs
Stellar mass @z=0
e.g., Marconi & Hunt 03
Star forming history vs
accretion history
Marconi+ 04
The X-Ray Background (XRB)
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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 0.5-2 keV, but 10-100 keV is the
best energy band to detect obscured AGNs including Compton thick
AGNs
Comastri+ 95
X-ray Spectra of Heavily Obscured AGNs
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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)
Log NH=24.25
Log NH=24.75
Log NH=25.25
Done+ (2003)
NGC 4945
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
Sample: 1371 AGNs
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survey
HEAO-1
ASCA
Chandra
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ROSAT/XMM/Chandra 1020
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N
49
142
160
flux limit reference of optical ID
1.7x10-11
Piccinotti+82, Grossan+82
3x10-14
Akiyama+00, Akiyama+03, Ishisaki+01
1.1x10-15
Barger+03, Szokoly+04, Zheng+ 04
1.1x10-16
Hasinger+ 05 and therin
(1)+(2) → Population Synthesis Model
The current status of X-ray surveys
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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
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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 0.5-1000 keV
The missing background is then be attributed to Compton thick
AGNs
Population synthesis model
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Integrated AGN emission from
our HXLF and the absorption
function (lower black) well
reproduces the broad band XRB
spectrum (blue) below 300 keV
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High energy cutoff must be
arround 500 keV in average
(red left: 400 keV,
red right: 600 keV)
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Presense of Compton-thick
AGNs estimated by Risaliti et al
(1999) is consistent with the XRB
spectrum (upper black)
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Reflection components are
important
(green: no reflection)
The observed XRB
spectrum
Ueda+ 2003
Compton thick AGNs or Compton reflection?
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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.
Observed XRB spectrum
Integrated spectrum
of type-1 AGNs
Compton-thick AGNs
Ueda+ 2003
0.5
1
10
100
(keV)
A big remaining issue:
The number density of Compton thick AGNs
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Significant contribution to the SMBH mass growth?
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“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
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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)
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At higher redshifts, little is known about the number density of
Compton thick AGNs
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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)
Evidence for Compton thick AGNs
in the local universe
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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)
log N log S relation above 10 keV
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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)
Ueda+ 03
Fraction of Compton thick AGNs
NeXT limit
~40-50% XRB
10-30 keV Survey
2-8 keV Survey
Summary
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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.