The iron K alpha line complex in Compton-thick Seyfert 2 galaxies

We calculate the intensities of the fluorescent and resonantly scattered iron K alpha lines in those active galactic nuclei (AGN) in which the primary radiation at the line energy is completely blocked by intervening matter along the line of sight (the so-called 'Compton-thick' Seyfert 2 galaxies). In the framework of unification models for Seyfert galaxies, both a 'cold' fluorescent line from the obscuring torus and highly ionized lines from the warm material responsible for the scattering and polarization of optical photons are expected. The neutral iron line from the obscuring torus has an equivalent width, with respect to the continuum reflected by the same matter, of order 1-2 keV, provided that the matter is Compton thick. The line emission from the warm material, in the optically thin regime for all relevant processes, is dominated by the resonantly scattered lines. However, for plausible physical conditions of this matter, the resonant lines are likely to be optically thick in their centres. In this case, the resonant line intensity can significantly decrease, and for tau(T) greater than or similar to 0.1 the fluorescent/recombination lines dominate the resonant lines. We present analytical formulae for the equivalent widths of iron lines in the optically thin case, and numerical results (obtained by means of Monte Carlo simulations) for the optically thick case. We apply the theoretical results to the most famous Compton-thick source, the Seyfert 2 galaxy NGC 1068, The large uncertainties in the continuum spectral shape and line equivalent widths prevent any definite conclusions, but the application can be regarded as an example of how the results presented in this paper could be used to estimate physical and geometrical quantities in obscured AGN.