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Author: Johannes Buchner Homepage: http://astrost.at/istics/ Further information: Robust and complete Demographics of Active Galactic NucleiAbstract: A robust determination of fraction of obscured and Compton-thick AGN is a long-standing issue in AGN research. We address this question using a multi-tiered survey of 2000 X-ray hard-band detected AGN covering well the redshift range of z=0.5-4, luminosities of L=10^42-46 erg/s and column densities of NH=10^20-26 cm^-2. We analyse the spectrum of each source in detail with a Bayesian fitting technique capable of handling also the low counts, and propagate errors in association and photometric redshift. We developed a new non-parametric method that reveals the distribution of the AGN population in luminosity, obscuration and redshift. We find the fraction of Compton-thick AGN to be 36%(+-8) averaged over cosmic time, while 75%(+-4) AGN are obscured. These fractions evolve with cosmic time, as AGN of different obscuration show distinct evolutionary patterns, with AGN around NH~10^23 cm^-2 evolving fastest. We study the luminosity dependence of the fraction of obscured and Compton-thick AGN in detail, and find that this relationship evolves over cosmic time. In my talk I will discuss models that can explain the large fraction of Compton-thick AGN as well as the luminosity-obscuration relationship. 
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