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 AGN 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 AGN 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) AGN 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 AGN increases from 0.45±0.10 at $z=0.25$ to 0.75±0.05 at $z=1.25$. Studies that select AGN 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 AGN (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGN 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 AGN is primarily a function of redshift. This fraction increases from about 20-30% at z=0.25 to about 50-70% at $z=1.5$.