1. INTRODUCTION

Most of the active galactic nuclei (AGN) in the local universe are obscured in the X-ray band by large amounts of gas and dust, which prevent the observation of their nuclear emission up to energies that depend on the amount of intrinsic absorption. If the X-ray obscuring matter has a column density which is equal to or larger than the inverse of the Thomson cross-section (N_{H T}^-1 1.5 × 10²⁴ cm^-2), then the source is called, by definition, "Compton thick". The cross-sections for Compton scattering and photoelectric absorption have approximately the same value for energies of order 10 keV, which can be considered as the low energy threshold for probing the Compton thick absorption regime. Indeed, if the column density does not exceed a value of order 10²⁵ cm^-2, then the nuclear radiation is visible above 10 keV, and the source is called mildly Compton thick. For higher column densities (heavily Compton thick), the entire high energy spectrum is down-scattered by Compton recoil and hence depressed over the entire X-ray energy range. The presence of Compton thick matter may be inferred through indirect arguments, such as the presence of a strong iron K line complex at 6.4 - 7 keV and the characteristic reflection spectrum.

The study of Compton thick sources is relevant for several reasons: (1) there is observational evidence that suggests that a large fraction of AGN in the local universe are obscured by Compton thick gas (Maiolino et al. 1998; Risaliti, Maiolino, & Salvati 1999a; Matt et al. 2000); (2) a sizable population of mildly Compton thick sources is postulated in all the AGN synthesis models for the X-ray background (XRB) in order to match the intensity peak of the XRB spectrum at about 30 keV. The absorbed luminosity eventually will be reemitted in the far-infrared (far-IR), making Compton thick sources potential contributors to the long wavelength background. Finally, accretion in the Compton thick AGN may contribute to the local black hole mass density.

Unfortunately, the most efficient energy range to search for mildly Compton thick sources is just above the highest energy accessible to the past and present generation of satellites with imaging capabilities for faint limiting fluxes. As a consequence, the search for Compton thick sources has been limited, so far, to the relatively bright fluxes accessible to the high energy detectors onboard BeppoSAX and RXTE.

In this chapter, we review the evidence for obscured AGN, with a special emphasis on the evidence for Compton thick sources. We then examine in some detail the contributions of Compton thick sources to the XRB.