DENSE, THIN CLOUDS AND REPROCESSED RADIATION IN THE CENTRAL REGIONS OF ACTIVE GALACTIC NUCLEI

The primary radiation generated in the central continuum-forming regio n of active galactic nuclei (AGN) can be reprocessed by very dense, sm all-scale clouds that are optically thin to Thomson scattering. In spi te of the extreme conditions expected to prevail in this innermost, ce ntral environment, the radiative clouds can survive and maintain cool temperatures relative to the ambient emitting region by means of confi nement by strong magnetic fields. Motivated by these ideas, we present a detailed quantitative study of such clouds, explicitly describing t he physical properties they can attain under thermal and radiative equ ilibrium conditions. We also discuss the thermal stability of the gas in comparison with that of other reprocessing material thought to resi de at larger distances from the central source. We construct a model t o predict the emergent spectra from a source region containing a total line-of-sight column density less than or similar to 10(23) cm(-2) of dense clouds which absorb and re-emit the primary radiation generated therein. Our predicted spectra show the following two important resul ts: (i) the reprocessed flux emitted at optical/ultraviolet (UV) energ ies is insufficient to account for the blue bump component in the obse rved spectra; and (ii) the amount of line radiation that is emitted is at least comparable to (and in many cases dominates) the continuum ra diation, The lines are extremely broad and tend to accumulate in the e xtreme ultraviolet (EUV), where they form a peak much more prominent t han that which is observed in the optical/UV. This result is supported by current observations, which indicate that the spectral energy dist ribution (SED) of radio-quiet AGN may indeed reach a maximum in the EU V band.