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

The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured active galactic nuclei (AGNs) at high accretion luminosities, L_X(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, N_H ≈ 10²⁴ cm^-2. The fraction of obscured Compton-thin (N_H = 10²²-10²⁴ cm^-2) AGNs is estimated to be ≈0.35 for luminosities L_X(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.