Invited talk at the conference X-ray Astronomy 1999: Stellar Endpoints, AGNs and the Diffuse X-ray Background

Abstract. The properties of the absorption in type 2, narrow line AGNs are reviewed by focusing on the X-ray indicators. I discuss the properties of the cold absorbing medium (the putative torus) and of the reprocessed components, as well as their implications for the unified model. The relation between optical classification and X-ray absorption is examined. The case of "fossil" AGNs, whose type 2 classification is not due to absorption effects, is also discussed. Although this review is mainly focused on nearby Seyfert 2 galaxies, I also shortly discuss the effects of absorption at higher luminosities and higher redshift and the implications for the X-ray background.

Keywords: galaxies: active - galaxies: Seyfert - galaxies: ISM - X-rays: galaxies

Table of Contents

• INTRODUCTION
• X-RAY ABSORPTION AS A DIAGNOSTIC OF THE TORUS
• The Distribution of N_H
• X-ray Absorption as a Probe of the Parsec Scale Torus
• X-ray Absorption as a Probe of the 100 pc Scale Torus
• Intermediate Type Seyferts
• Dual Absorbers
• X-RAY ABSORPTION AND GALAXY MORPHOLOGY
• THE RELATION BETWEEN OPTICAL AND X-RAY ABSORPTION
• PIERCING COMPLETELY HIDDEN AGNs
• FOSSIL ACTIVE NUCLEI
• IMPLICATIONS FOR THE X-RAY BACKGROUND
• REFERENCES