The Cosmological Evolution of AGN X-ray Luminosity Function

1. The Cosmological Evolution of AGN X-ray Luminosity Function Yoshihiro Ueda (Kyoto University) Guenther Hasinger (MPE) Takamitsu Miyaji (Carnegie Mellon)

2. The X-ray Background (XRB) • The XRB is the integrated emission from all the AGNs in the universe, telling us the formation history of supermassive black holes. • At least below 6-8 keV it is now almost completely resolved into discrete sources, mainly AGNs. Log N log S relations (2-10 keV) 1 deg Kushino+ 02 Subaru-XMM Deep Survey fields

3. The necessity of hard band surveys • 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). • Hard band surveys (above 2 keV) are indispensable to detect obscured AGNs. • Sensitive surveys below 10 keV, currently available, can provide us with a complete picture of “Compton thin” AGN (log NH<24) in the universe. • It is critical to establish the cosmological evolution of Compton thinAGNs, in order to evaluate the role of "Compton thick" AGNs in theaccretion history, a major theme in next generation X-ray astronomy. Comastri+ 95

4. Solving the XRB origin Two major elements 1. X-ray Luminosity function (XLF) • The co-moving spatial number density of AGNs as a function of luminosity and redshift • Surveys in the 0.5-2 keV band: mostly type1 AGNs (log NH <22) • Surveys in the 2-10 keV band: type1+type2 Compton thin AGNs (log NH <24) 2. Absorption function (NH function) • Probability distribution function of absorption column density of an AGN at a given luminosity and a redshift, giving the fraction of absorbed (log NH >22) AGNs. • A fundmental quantity for understanding of the AGN phenomena • Is “unified scheme” correct? • Evolution? • Hard band sample is required

5. < 22 23 24 log NH NH distribution of AGNs in the local universe • The Swift/BAT and Integral hard X-ray surveysabove 15 keV show that absorbed AGNs are indeed a major population. The fraction of absorbed sources with (log NH > 22) is 0.5-0.6. • The results of softer-band surveys are consistent with the Swift result after correcting for selection bias against absorbed sources HEAO1 (2-10 keV) Shinozaki+ 2006 RXTE/ASM (3-20 keV) Szanov & Revnivtsev 2004 Swift/BAT (15-200 keV) Markwardt+ 2005 20 22 24 26 log NH • 21 22 23 24 • log NH

6. NH distribution of AGNsin the high redshift universe • Due to the statistical fluctuation in photon counts, an unabsorbed AGN easily appear as an absorbed source if we rely on the best-fit NH. More problematic at higher redshift. (see also Akylas+06) • In small photon statistics data (especially in Chandra surveys), evaluation of systematic errors in the observed NH distribution is indispensable Simulated histogram of NH distribution for an “unabsorbed” AGN at z=2 Observed NH distribution of the CDFS Hard-band selected AGNs Tozzi+ 2005

7. Absorption Fraction • The most straightforward way to avoid this is to use only sources that have spectral information with sufficient photon statistics. XMM data are particularly useful for Chandra deep survey sources. • Our present analysis: 1. HEAO1 + ASCA/XMM follow-up (Piccinotti+82, Shinozaki+06)2. XMM Hard Bright Sample (Caccianiga+ 04)3. XMM Lockman Hole 800 ks (Hasinger+02, Matteos+05)4. CDFS + XMM 400 ks (Giacconi+02, Streblyanska+06 in prep) • While the luminosity dependence of absorption fraction is significant, its redshift dependence is not (Hasinger’s talk this morning)

8. Construction of an ultimate XLFcombined analysis of the HXLF and SXLF • There is still room for improvement of the population synthesis model after Ueda+ 03 (e.g., more precise modeling of LF and absorption function, Hasinger’s talk) • Best constrain the rest-frame 2-10 keV LF of all Compton-thin AGNs using all the heritage of X-ray surveys with various depth, width, and energy bands performed up to date. • While hard band surveys above 2 keV are indispensable to detect type-2 AGNs at low redshifts, soft band (0.5-2 keV) surveys are also effective to detect type-2 AGNs at high redshifts thanks to the “K-correction” effect. Sensitivities achieved by X-ray reflecting mirrors are generally better in the soft band than in the hard band, leading to a deeper flux limit even for type 2 AGNs. • Utilize only samples with high identification completeness (>90%)

9. The analysis method • Maximum likelihood method applied to the most direct observationalquantities i.e.,list of count rate(flux) and redshift without any correction. • For a given model of luminosity function plus absorption function, we can calculate an expected count rate vs redshift distribution for each survey, by fully taking account of the energy response of instruments used for the survey.Spectral shape of an AGN and its variancecan be incorporated into the input model. • Find a solution that maximize the total probability of finding theobserved quantities. Soft and hard bands can be regarded as statistically independent measurement even for common sources. • In principle, luminosity function and absorption function can beconstrained simutaneously.

10. Sample: 1514 detections Survey N flux limit • HEAO-1 49 1.7x10-11 • ASCA MSS/LSS 125 1x10-13 • XMM LH 84 5x10-15 • CLASXS, CDFN/S 208 1.1x10-15 • ROSAT/XMM/Chandra 1048 1.1x10-16

11. Redshift distribution Soft band detected sample Hard band detected sample -1.5 -1 -0.5 1 1.5 -1.5 -1 -0.5 1 1.5 Log z Log z

12. Log N log S relations Hard band detected sample Soft band detected sample

13. HXLF The latest rest-frame 2-10 keV XLF of all Compton-thin AGNs, showing the “LDDE” behavior.

14. The AGN number density as a function of redshift • As previously found by Ueda+03 (type1+2) and Hasinger+05 (type 1), the cosmological evolution of the whole AGN is described with a luminosity dependent density evolution (LDDE) where the cut-off redshift increases with the luminosity • Luminous AGNs have a density peak earlier in the cosmic time than less luminous AGNs. • By assuming L~M, more massive SMBHs formed earlier than less massive SMBHs (“anti-hierarchical evolution” or “down sizing”)

15. Predicted XRB spectrum • The XRB intensity at 10 keV is ~10 % lower than the previous model, which does not utilize the CDFS sample This work (only Compton thin AGNs) Ueda+ 03 (only Compton thin AGNs)

16. Summary • The fraction of X-ray absorbed-AGN decreses with luminosity, while its redshift dependence is not significant. Fully consistent with the result from classification utilizing optical spectra (Hasinger’s talk). • An ultimate 2-10 keV XLF of all Compton-thin AGNs is being derived, confirming the “down-sizing” nature of BH growth. • The XRB synthesis model is close to its finalization, although there still remain issues: • The true number density of Compton thick AGNs, which is coupled with an assumption of broad band spectra of type1 and type2 AGNs (especially the amount of reflection) →sensitive hard X-ray (>10 keV) surveys are imortant.