X-ray constraints on the fraction of obscured AGN 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, $L_X (\rm 2-10 \, keV) > 10^{44}
\, erg \,s ^{-1}$, and out to redshift $z\approx1.5$. The sample covers an area
of about $\rm14\,deg^2$ 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, $\rm N_H\approx10^{24}\,cm^{-2}$. The fraction
of obscured Compton-thin ($\rm N_H=10^{22}-10^{24}\,cm^{-2}$) AGN is estimated
to be $\approx0.35$ for luminosities $L_X(\rm 2-10\,keV)>10^{44}\,erg\,s^{-1}$
independent of redshift. For less luminous sources the fraction of obscured
Compton-thin AGN increases from $0.45\pm0.10$ at $z=0.25$ to $0.75\pm0.05$ 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 $z=1.5$.
