X-ray observations of active galactic nuclei in the cosmological context

We review recent discoveries of obscured Active Galactic Nuclei at significant redshifts with X-ray observations. Most, if not all, the X-ray luminous galaxies found at faint X-ray fluxes could in fact be obscured AGN, where the obscuring material is also forming stars copiously. Given that AGN of various types appear to dominate the source populations at all X-ray fluxes, we also discuss their clustering properties. Current results imply that the population of X-ray selected AGN is not strongly biased, and therefore X-ray observations can be used to trace the distribution of matter out to redshifts ∼ 2.