Observations of Obscured Black Holes

Yoshihiro UedaYoshihiro Ueda (Department of Astronomy, Kyoto University) (Department of Astronomy, Kyoto University)

Search for obscured black holes Search for obscured black holes

Nearly every present-day galaxy contains a BH in Nearly every present-day galaxy contains a BH in its centre with a mass proportional to the spheroid its centre with a mass proportional to the spheroid mass, indicating a tight link between the BH and mass, indicating a tight link between the BH and star formation: star formation: SMBH is a key ingredient of the SMBH is a key ingredient of the universeuniverse

Most AGNs are “Most AGNs are “obscuredobscured” (cannot always be ” (cannot always be distinguished or recognized in other wavelengths). distinguished or recognized in other wavelengths). Hard X-ray observations are the most Hard X-ray observations are the most straightforward approach to detect this population straightforward approach to detect this population without selection biaseswithout selection biases

In fact, massive star forming galaxies contain In fact, massive star forming galaxies contain rapidly growing BHs heavily obscured by dust rapidly growing BHs heavily obscured by dust (submilimeter galaxies at z~2, Alexander et al. (submilimeter galaxies at z~2, Alexander et al. 2005; ULIRGs at z~0, Imanishi et al. 2006). This is 2005; ULIRGs at z~0, Imanishi et al. 2006). This is consistent with the “co-evoluton” scenario.consistent with the “co-evoluton” scenario.

Co-evolution of galaxy and super Co-evolution of galaxy and super massive black holes in galactic centresmassive black holes in galactic centres

e.g., Marconi & Hunt 03e.g., Marconi & Hunt 03

Star forming history vs Star forming history vs accretion historyaccretion history

Marconi+ 04Marconi+ 04

BH mass vs BH mass vs Stellar mass @Stellar mass @z=0z=0

The X-Ray Background (XRB)The X-Ray Background (XRB)

The XRB is the integrated eission from all the AGNs in the The XRB is the integrated eission from all the AGNs in the universe, telling us the formation history of supermassive universe, telling us the formation history of supermassive black holes. black holes.

The energy density peaks at ~ 30 keVThe energy density peaks at ~ 30 keV The shape of the XRB indicates that The shape of the XRB indicates that most of the AGNs are most of the AGNs are

obscuredobscured (such as Seyfert 2; Awaki et al. 1993) (such as Seyfert 2; Awaki et al. 1993) The 2-10 keV band is much better than 0.5-2 keV, but 10-100 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 keV is the best energy band to detect obscured AGNs including Compton thick AGNs Compton thick AGNs

Comastri+ 95

X-ray Spectra of Heavily Obscured X-ray Spectra of Heavily Obscured AGNsAGNs

Compton thick AGNs:Compton thick AGNs: N NHH>10>1024 24 cmcm-2-2 show complex spectra show complex spectra as a function of column densityas a function of column density

Reflection/scattered component can be detected below 10 Reflection/scattered component can be detected below 10 keV but only limited information can be drawn (e.g.,no keV but only limited information can be drawn (e.g.,no intrinsic luminosity)intrinsic luminosity)

NGC 4945

Done+ (2003)Wilman & Fabian (1999)

Log NH=24.25

Log NH=24.75

Log NH=25.25

Subaru-XMM Deep Survey fields

1 deg

Log N log S relations (2-10 keV) Kushino+ 02

What are known from X-ray surveys below What are known from X-ray surveys below 10 keV10 keV

Sample: 1371 AGNsSample: 1371 AGNs survey N flux limit reference of optical IDsurvey N flux limit reference of optical ID HEAO-1 49 1.7x10HEAO-1 49 1.7x10-11 -11 Piccinotti+82, Grossan+82 Piccinotti+82, Grossan+82 ASCA 142 3x10ASCA 142 3x10-14 -14 Akiyama+00, Akiyama+03, Ishisaki+01 Akiyama+00, Akiyama+03, Ishisaki+01 Chandra 160 1.1x10Chandra 160 1.1x10-15 -15 Barger+03, Szokoly+04, Zheng+ 04 Barger+03, Szokoly+04, Zheng+ 04

ROSAT/XMM/Chandra 1020 1.1x10ROSAT/XMM/Chandra 1020 1.1x10-16-16 Hasinger+ 05 and therin Hasinger+ 05 and therin

The current status of X-ray surveysThe current status of X-ray surveys The XRB below ~ 6 keV has been almost completely The XRB below ~ 6 keV has been almost completely

resolved and identified and hence is well understood.resolved and identified and hence is well understood. Howerver, even with the deepest Chandra/XMM surveys a Howerver, even with the deepest Chandra/XMM surveys a

significant fraction of the XRB remains unresolved above significant fraction of the XRB remains unresolved above 6 keV (Worsley et al. 2004). Above 10 keV only 1 percent 6 keV (Worsley et al. 2004). Above 10 keV only 1 percent of the XRB is resolved at present. of the XRB is resolved at present.

Extrapolation has to be made beyond the observational Extrapolation has to be made beyond the observational results.results.

Population synthesis model reproducing the XRB Population synthesis model reproducing the XRB spectrumspectrum

Given the luminosity function and absorption function Given the luminosity function and absorption function determined determined below below 10 keV, we predict contribution of 10 keV, we predict contribution of Compton-thin AGNs to the background Compton-thin AGNs to the background aboveabove 10 keV with 10 keV with an assumption an assumption of a broad band spectrum over 0.5-1000 of a broad band spectrum over 0.5-1000 keVkeV

The missing background is then be attributed to Compton The missing background is then be attributed to Compton thick AGNsthick AGNs

(1)+(2) →(1)+(2) → Population Synthesis ModelPopulation Synthesis Model

Population synthesis modelPopulation synthesis model

Integrated AGN emission Integrated AGN emission from our HXLF and the from our HXLF and the absorption function absorption function (lower (lower black)black) well reproduces the well reproduces the broad band XRB spectrumbroad band XRB spectrum (blue)(blue) below 300 keV below 300 keV

High energy cutoff must be High energy cutoff must be arround 500 keV in average arround 500 keV in average

(red left: 400 keV, (red left: 400 keV, red right: 600 keV)red right: 600 keV)

Presense of Compton-thick Presense of Compton-thick AGNs estimated by AGNs estimated by Risaliti et Risaliti et al (1999)al (1999) is consistent with is consistent with the XRB spectrum the XRB spectrum (upper (upper black)black)

Reflection components are Reflection components are important important

(green: no reflection)(green: no reflection)

The observed XRB spectrum

Ueda+ 2003

Compton thick AGNs or Compton Compton thick AGNs or Compton reflection?reflection?

The fraction of Compton thick AGNs, introduced to reproduce the The fraction of Compton thick AGNs, introduced to reproduce the intensity XRB spectrum at 30 keV, is coupled with the amount of intensity XRB spectrum at 30 keV, is coupled with the amount of reflection component (assumed to be Ω=2πfor both type-1 and reflection component (assumed to be Ω=2πfor both type-1 and type-2 AGNs) type-2 AGNs)

Precise study of broad band spectra of neaby AGNs (especially Precise study of broad band spectra of neaby AGNs (especially type-2 AGNs) is crucial. Suzaku observations are important.type-2 AGNs) is crucial. Suzaku observations are important.

Observed XRB spectrum Integrated spectrum of type-1 AGNs

Compton-thick AGNs

0.5 1 10 100 (keV)

Ueda+ 2003

A big remaining issue:A big remaining issue:The number density of Compton thick AGNsThe number density of Compton thick AGNs

Significant contribution to the SMBH mass growth?Significant contribution to the SMBH mass growth? ““AGN relic” black hole mass function can be calculated from AGN relic” black hole mass function can be calculated from

the luminosity function of the “whole AGNs” including Compton the luminosity function of the “whole AGNs” including Compton thick onesthick ones

For instance, Marconi et al. (2004) have to assume 0.6 times For instance, Marconi et al. (2004) have to assume 0.6 times additional Compton thick AGNs as many as Compton thin ones additional Compton thick AGNs as many as Compton thin ones to reproduce the local BH mass function to reproduce the local BH mass function

In the local universe the number density of Compton thick In the local universe the number density of Compton thick AGNs may be comparable to or even larger than Compton AGNs may be comparable to or even larger than Compton thin AGNs (Maiolino et al. 2003)thin AGNs (Maiolino et al. 2003)

At higher redshifts, little is known about the number density At higher redshifts, little is known about the number density of Compton thick AGNs of Compton thick AGNs Mid IR + radio selection of type-2 QSOs at z~2 implys twice as Mid IR + radio selection of type-2 QSOs at z~2 implys twice as

large number density as Compton large number density as Compton thinthin type-2 QSOs (Martines- type-2 QSOs (Martines-Sansigre et al. 2006)Sansigre et al. 2006)

Evidence for Compton thick AGNsEvidence for Compton thick AGNs in the local universe in the local universe

A population of infrared galaxies A population of infrared galaxies that do not show AGN signatures in that do not show AGN signatures in their optical spectra are found to be their optical spectra are found to be Compton thick AGNs by Chandra Compton thick AGNs by Chandra follow-up follow-up ((optically elusive AGNsoptically elusive AGNs) )

The number density is comparable The number density is comparable to that of Seyfert 2 galaxiesto that of Seyfert 2 galaxies

Their nucleus is completely hidden Their nucleus is completely hidden so that no narrow-line region form?so that no narrow-line region form?

Swift/BAT or INTEGRAL surveys will Swift/BAT or INTEGRAL surveys will give a definete answer for this at give a definete answer for this at

least for those with log 24 <least for those with log 24 <NNH<25<25

Maiolino et al. (2003)

log N log S relation above 10 log N log S relation above 10 keV keV

If we simply extrapolate the If we simply extrapolate the NNH function below log function below log NNH=24 to log =24 to log NNH=26 based on the Ueda 2003 =26 based on the Ueda 2003 model, thenmodel, then

The fraction of Compton thick AGNs is predicted The fraction of Compton thick AGNs is predicted to be ~10% (Fx=1e-11) to ~25% (Fx=1e-16)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 AGNsUeda+ 03

SummarySummary There is growing evidence for the presence of a There is growing evidence for the presence of a

large population of Compton thick AGNs in the large population of Compton thick AGNs in the universe.universe.

We have We have not fullynot fully understand the XRB origin yet. understand the XRB origin yet. The population synthesis model is being almost The population synthesis model is being almost established below 6-8 keV. However, we have to established below 6-8 keV. However, we have to note that there are a few critical assumptions note that there are a few critical assumptions are made when extrapolating it to above 8 keV.are made when extrapolating it to above 8 keV.

To fully understand the accretion history of the To fully understand the accretion history of the universe, it is critical to reveal the evolution of universe, it is critical to reveal the evolution of Compton thick AGNs. Compton thick AGNs.

Sensitive hard X-ray surveys above 10 keV with Sensitive hard X-ray surveys above 10 keV with various depths and widths, as done at energies various depths and widths, as done at energies below 10 keV, are the only way to unveil this below 10 keV, are the only way to unveil this problem. problem.