OPTICALLY THIN THERMAL EMISSION FROM COLD CLOUDS IN ACTIVE GALACTIC NUCLEI

The thin thermal emission from an ensemble of cold condensations embed ded in a hot plasma is investigated. The temperature distribution insi de the cloud is derived by integrating the energy equation where heati ng, radiative losses and thermal conduction balance each other. The pr esence of the thermal conduction term, which constitutes the main diff erence with respect to the previous models, determines the scale lengt h of the cloud and makes the cold cloud core very narrow while the sur rounding shell, only slightly cooler than the external medium, is very extended. It follows that the emission from the cold core is negligib le with respect to that of the extended shell, which in turn can be mo re luminous than the surrounding medium only if a strong negative pres sure gradient is introduced. Hence, the emission from such a configura tion does not show a thermal peak characterized by a temperature value ranging from 10(5) to 10(6) K as observed in the active galactic nucl ei (AGN) spectra. These conclusions are independent of the selected he ating and loss functions and also the introduction of a magnetic field cannot change them. If evolving structures can be a viable solution t o this problem is also briefly analyzed.