Obscured active galactic nuclei: the hidden side of the X-ray Universe.

Most active galactic nuclei (AGN) are 'obscured', i.e. the nucleus is hiding behind a screen of absorbing material. The advantage of having the nucleus obscured is to make easier the observations of those emission components which originate in circumnuclear matter outside the absorbing regions, because in this case they are not outshone by the nuclear emission. This is particularly important in X-rays, where spatial resolution is (with the notable exception of Chandra) poorer than in the optical, and the study of circumnuclear regions is often based on spectral analysis only. The properties of circumnuclear matter, in the light of recent high spectral and/or angular resolution Chandra and XMM-Newton observations, are reviewed and discussed in the framework of the unification model. Recent discoveries of X-ray obscured Seyfert 1, and of X-ray loud but optically normal galaxies, are calling for a revision of the unification model. Obscured AGN have also a cosmological relevance. Not only are they a fundamental ingredient of synthesis models of the cosmic X-ray background (XRB), but provide a link between the XRB and the cosmic infrared background.