Dense, thin clouds and reprocessed radiation in the central regions of active galactic nuclei

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Abstract
The primary radiation generated in the central continuum-forming region of active galactic nuclei (AGN) can be reprocessed by very dense, small-scale clouds that are optically thin to Thomson scattering. In spite of the extreme conditions expected to prevail in this innermost, central environment, the radiative clouds can survive and maintain cool temperatures relative to the ambient emitting region by means of confinement by strong magnetic fields. Motivated by these ideas, we present a detailed quantitative study of such clouds, explicitly describing the physical properties they can attain under thermal and radiative equilibrium conditions. We also discuss the thermal stability of the gas in comparison with that of other reprocessing material thought to reside at larger distances from the central source. We construct a model to predict the emergent spectra from a source region containing a total line-of-sight column density ≲1023 cm−2 of dense clouds which absorb and re-emit the primary radiation generated therein. Our predicted spectra show the following two important results: (i) the reprocessed flux emitted at optical/ ultraviolet (UV) energies is insufficient to account for the blue bump component in the observed spectra; and (ii) the amount of line radiation that is emitted is at least comparable to (and in many cases dominates) the continuum radiation. The lines are extremely broad and tend to accumulate in the extreme ultraviolet (EUV), where they form a peak much more prominent than that which is observed in the optical/UV. This result is supported by current observations, which indicate that the spectral energy distribution (SED) of radio-quiet AGN may indeed reach a maximum in the EUV band.
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