Review for "Supermassive Black Holes in the Distant Universe", Ed. A. J. Barger, Kluwer Academic
astro-ph/0403693

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Abstract. The spectrum of the hard X-ray background records the history of accretion processes integrated over the cosmic time. Several pieces of observational and theoretical evidence indicate that a significant fraction of the energy density is obscured by large columns of gas and dust. The absorbing matter is often very thick, with column densities exceeding N_H 1.5 × 10²⁴ cm^-2, the value corresponding to unity optical depth for Compton scattering. These sources are called "Compton thick" and appear to be very numerous, at least in the nearby universe. Although Compton thick Active Galactic Nuclei (AGN) are thought to provide an important contribution to the overall cosmic energy budget, their space density and cosmological evolution are poorly known. The properties of Compton thick AGN are reviewed here, with particular emphasis on their contributions to the extragalactic background light in the hard X-ray and infrared bands.

Table of Contents

• INTRODUCTION
• ABSORPTION DISTRIBUTION IN THE LOCAL UNIVERSE
• COMPTON THICK AGN AND THE XRB
• DISTANT OBSCURED AGN
• BLACK HOLE MASS DENSITY OF COMPTON THICK SOURCES
• CONTRIBUTION TO THE FAR-IR BACKGROUND
• CONCLUSIONS AND FUTURE PROSPECTS
• REFERENCES

¹ Partially supported by the Italian Space Agency (ASI) under grants I/R/073/01 and I/R/057/02, by the MIUR grant Cofin-03-02-23 and by the INAF grant 270/2003.