Obscured AGNs are thought to be a key ingredient of the hard X-ray background (XRB, Setti & Woltjer 1991, Comastri et al. 1995). This is discussed in detail in this volume by other authors (eg. Comastri). However, I wish to emphasize that some of the issues discussed in this review might have implications for our understanding of the origin of the hard XRB.
The distribution of NH represents the main set of free parameters in the XRB synthesis models. The NH distribution presented in sect. 2.1 can be used to freeze this set of parameters, under the assumption that the distribution does not evolve with redshift. Detailed models that take into account this constraint are presented in Gilli et al. (1999, 2000b): although the shape and power of the XRB is well reproduced, the observed number counts in the hard X-rays seem to require an evolution of the obscured AGNs faster than for the unobscured population. This result is also related to the finding that non-axisymmetric morphologies increase the obscuration of the active nuclei (sect. 3). Indeed, the latter effect should have an impact on the XRB, since galaxies at high redshift are characterized by a higher rate of distorted/irregular morphologies.
The deficiency of dust absorption, with respect to the X-ray absorption, especially at high luminosities, implies a possible mismatch between optical and X-ray classification of the sources contributing to the hard X-ray background. In particular, some of the type 2 QSOs, which are expected to make most of the hard XRB, could be optically ``masked'' as type 1 QSOs and already present in optical surveys. As mentioned in Sect. 4, the early Chandra surveys have identified only a few objects of this class, at variance with what found in radio and past hard X-surveys (Sect. 4). Possibly the discrepancy is due to the sensitivity of Chandra which peaks in the soft X-ray band and, therefore, probably biases the surveys in favor of little absorbed sources. To properly tackle this issue we should wait for the identification of a larger number of Chandra sources (especially at fainter fluxes, where the fraction of absorbed sources is higher) and the results of the surveys that are being performed with XMM, whose sensitivity is much more uniform up to ~ 7 keV.
Many of the new results presented in this paper were obtained in collaboration with R. Gilli, A. Marconi, G. Risaliti and M. Salvati. This work was partially supported by the Italian Space Agency (ASI) through the grant ARS-99-15 and by the Italian Ministry for University and Research (MURST) through the grant Cofin-98-02-32.