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Astron. Astrophys. 335, 881-893 (1998)

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Optically thin thermal emission from cold clouds in active galactic nuclei

G. Torricelli-Ciamponi 1, 2 and T.J.-L. Courvoisier 1, 3

1 Geneva Observatory, ch. des Maillettes 51, CH-1290 Sauverny, Switzerland
2 Osservatorio di Arcetri, Largo E.Fermi 5, I-50125 Firenze, Italy
3 ISDC, ch. d'Ecogia 16, CH-1290 Versoix, Switzerland

Received 30 September 1997 / Accepted 6 April 1998


The thin thermal emission from an ensemble of cold condensations embedded in a hot plasma is investigated. The temperature distribution inside the cloud is derived by integrating the energy equation where heating, radiative losses and thermal conduction balance each other. The presence of the thermal conduction term, which constitutes the main difference with respect to the previous models, determines the scale length of the cloud and makes the cold cloud core very narrow while the surrounding shell, only slightly cooler than the external medium, is very extended. It follows that the emission from the cold core is negligible with respect to that of the extended shell, which in turn can be more luminous than the surrounding medium only if a strong negative pressure gradient is introduced. Hence, the emission from such a configuration does not show a thermal peak characterized by a temperature value ranging from [FORMULA] to [FORMULA] as observed in the active galactic nuclei (AGN) spectra. These conclusions are independent of the selected heating and loss functions and also the introduction of a magnetic field cannot change them. If evolving structures can be a viable solution to this problem is also briefly analyzed.

Key words: radiation mechanisms: thermal – ultraviolet: galaxies – galaxies: nuclei – galaxies: Seyfert – galaxies: quasars: general

Send offprint requests to: G. Torricelli (Arcetri)

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998