AGN Dusty Tori. I. Handling of Clumpy Media

Accordingtounifiedschemesof activegalactic nuclei(AGNs), thecentralengine issurroundedbydusty,optically thickcloudsinatoroidalstructure.Wehaverecentlydevelopedaformalismthatforthefirsttimetakesproperaccount of the clumpy nature of the AGN torus. We now provide a detailed report of our findings in a two-paper series. Here wepresentourgeneralformalismforradiativetransferinclumpymediaandconstructitsbuildingblocksfortheAGN problem—the source functions of individual dusty clouds heated by the AGN radiation field. We show that a fundamental difference from smooth density distributions is that in a clumpy medium, a large range of dust temperatures coexist at the same distance from the radiation central source. This distinct property explains the low dust temperatures found close to the nucleus of NGC 1068 in 10� m interferometric observations. Wefind that, irrespective of the overallgeometry,aclumpydustdistributionshowsonlymoderate variationinitsspectralenergydistribution, andthe 10� mabsorptionfeatureisneverdeep.Furthermore,theX-rayattenuatingcolumndensityiswidelyscatteredaround the column density that characterizes the IR emission. All of these properties are characteristic of AGN observations. The assembly of clouds into AGN tori and comparison with observations are presented in the companion paper. Subject headingg dust, extinction — galaxies: active — galaxies: Seyfert — infrared: general — quasars: general — radiative transfer