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Obscuration-dependent Evolution of Active Galactic Nuclei

Buchner, Johannes and Georgakakis, Antonis and Nandra, Kirpal and Brightman, Murray and Menzel, Marie-Luise and Liu, Zhu and Hsu, Li-Ting and Salvato, Mara and Rangel, Cyprian and Aird, James and Merloni, Andrea and Ross, Nicholas (2015) Obscuration-dependent Evolution of Active Galactic Nuclei. Astrophysical Journal, 802 (2). Art. No. 89. ISSN 0004-637X. doi:10.1088/0004-637X/802/2/89.

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We aim to constrain the evolution of active galactic nuclei (AGNs) as a function of obscuration using an X-ray-selected sample of ~2000 AGNs from a multi-tiered survey including the CDFS, AEGIS-XD, COSMOS, and XMM-XXL fields. The spectra of individual X-ray sources are analyzed using a Bayesian methodology with a physically realistic model to infer the posterior distribution of the hydrogen column density and intrinsic X-ray luminosity. We develop a novel non-parametric method that allows us to robustly infer the distribution of the AGN population in X-ray luminosity, redshift, and obscuring column density, relying only on minimal smoothness assumptions. Our analysis properly incorporates uncertainties from low count spectra, photometric redshift measurements, association incompleteness, and the limited sample size. We find that obscured AGNs with N_H > 10^(22) cm^(–2) account for 77^(+4_(-5)% of the number density and luminosity density of the accretion supermassive black hole population with L_X > 10^(43) erg s^(–1), averaged over cosmic time. Compton-thick AGNs account for approximately half the number and luminosity density of the obscured population, and 38^(+8)_(-7)% of the total. We also find evidence that the evolution is obscuration dependent, with the strongest evolution around N_H ≈ 10^(23) cm^(–2). We highlight this by measuring the obscured fraction in Compton-thin AGNs, which increases toward z ~ 3, where it is 25% higher than the local value. In contrast, the fraction of Compton-thick AGNs is consistent with being constant at ≈35%, independent of redshift and accretion luminosity. We discuss our findings in the context of existing models and conclude that the observed evolution is, to first order, a side effect of anti-hierarchical growth.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Buchner, Johannes0000-0003-0426-6634
Nandra, Kirpal0000-0002-7150-9192
Brightman, Murray0000-0002-8147-2602
Liu, Zhu0000-0002-8968-7050
Salvato, Mara0000-0001-7116-9303
Aird, James0000-0003-1908-8463
Ross, Nicholas0000-0003-1830-6473
Additional Information:© 2015 American Astronomical Society. Received 2014 October 7; accepted 2014 December 21; published 2015 March 26. J.B. acknowledges financial support through a Max Planck society stipend. We thank the builders and operators of Chandra X-ray Observatory and XMM-Newton. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application package CIAO and Sherpa. Additionally, the CosmoloPy (, BXA (, and pymultinest ( software packages were used. A.G.E. acknowledges the THALES project 383549, which is jointly funded by the European Union and the Greek Government in the framework of the program “Education and Lifelong Learning.” Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III Web site is SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration.
Funding AgencyGrant Number
Max Planck SocietyUNSPECIFIED
European Research Council (ERC)383549 THALES
Alfred P. Sloan FoundationUNSPECIFIED
Participating InstitutionsUNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Subject Keywords:galaxies: active; quasars: supermassive black holes; surveys; X-rays: galaxies
Issue or Number:2
Record Number:CaltechAUTHORS:20150515-151548160
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Official Citation:Obscuration-dependent Evolution of Active Galactic Nuclei Johannes Buchner et al. 2015 ApJ 802 89
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:57572
Deposited On:15 May 2015 23:34
Last Modified:10 Nov 2021 21:52

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