X-ray constraints on the fraction of obscured active galactic nuclei at high accretion luminosities

A. Georgakakis, M. Salvato, Z. Liu, J. Buchner, W. N. Brandt, T. Tasnim Ananna, A. Schulze, Yue Shen, S. LaMassa, K. Nandra, A. Merloni, I. D. McGreer

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Abstract

The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured active galactic nuclei (AGNs) at high accretion luminosities, LX(2-10 keV) ≳ 1044 erg s-1, and out to redshift z ≈ 1.5. The sample covers an area of about 14 deg2 and provides constraints on the space density of powerful AGNs over a wide range of neutral hydrogen column densities extending beyond the Compton-thick limit, NH ≈ 1024 cm-2. The fraction of obscured Compton-thin (NH = 1022-1024 cm-2) AGNs is estimated to be ≈0.35 for luminosities LX(2-10 keV) > 1044 erg s-1, independent of redshift. For less luminous sources, the fraction of obscured Compton-thin AGNs increases from 0.45 ± 0.10 at z = 0.25 to 0.75 ± 0.05 at z = 1.25. Studies that select AGNs in the infrared via template fits to the observed spectral energy distribution of extragalactic sources estimate space densities at high accretion luminosities consistent with the XMM-XXL constraints. There is no evidence for a large population of AGNs (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGNs as a function of accretion luminosity, column density and redshift. The fraction of XMM-XXL sources that lie within the mid-infrared colour wedges defined in the literature to select AGNs is primarily a function of redshift. This fraction increases from about 20-30 per cent at z = 0.25 to about 50-70 per cent at z = 1.5.

Original languageEnglish (US)
Pages (from-to)3232-3251
Number of pages20
JournalMonthly Notices of the Royal Astronomical Society
Volume469
Issue number3
DOIs
StatePublished - Aug 11 2017

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Georgakakis, A., Salvato, M., Liu, Z., Buchner, J., Brandt, W. N., Tasnim Ananna, T., Schulze, A., Shen, Y., LaMassa, S., Nandra, K., Merloni, A., & McGreer, I. D. (2017). X-ray constraints on the fraction of obscured active galactic nuclei at high accretion luminosities. Monthly Notices of the Royal Astronomical Society, 469(3), 3232-3251. https://doi.org/10.1093/mnras/stx953