Observations of Obscured Black Holes

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. Co-evolution of galaxy and super massive black holes in galactic centres BH mass vs Stellar mass @z=0 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 0.5-2 keV, but 10-100 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-1 49 1.7x10-11 Piccinotti+82, Grossan+82 • ASCA 142 3x10-14 Akiyama+00, Akiyama+03, Ishisaki+01 • Chandra 160 1.1x10-15 Barger+03, Szokoly+04, Zheng+ 04 • ROSAT/XMM/Chandra 1020 1.1x10-16 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 0.5-1000 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: 400 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. Integrated spectrum of type-1 AGNs Compton-thick AGNs 0.5 1 10 100 (keV) 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. 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. Fraction of Compton thick AGNs NeXT limit ~40-50% XRB 10-30 keV Survey 2-8 keV Survey 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) 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.