The effect of an attenuated continuum on the coronal line spectrum of NGC 1068 and the Circinus galaxy

We present photoionization models of the optical and IR coronal Line spectr um in NGC 1068 and the Circinus galaxy. The line fluxes have been calculate d using (a) a non-thermal (nuclear) continuum source and (b) the nbn-therma l continuum plus a UV bump due to a stellar cluster. We take into account t he effect of attenuation of these continua by gas with column density 10(22 ) cm(-2) located between the nucleus-and the coronal line region. The calcu lated coronal line ratios are in a good agreement with those observed in NG C 1068 for a model in which about 40% of the line emission comes from gas i lluminated by unattenuated, non-thermal continuum, and about 60% from gas i lluminated by attenuated, non-thermal continuum. The electron density of th e coronal Line emitting gas in NGC 1068 is found to be 10(4) cm(-3). In the Circinus galaxy the coronal line emission comes from gas with-electron den sity 10(3) cm(-3) illuminated entirely by attenuated, non-thermal continuum . The derived ionization parameters for both coronal line regions are very Similar, but the different densities imply a higher ionizing Photon flux in NGC 1068, consistent with the higher observed excitation state of the Line emitting gas in that galaxy. A possible geometry of the coronal line regio n of both galaxies is discussed, in which the distribution of the attenuati ng gas may be strongly affected by the relative strength of the nuclear rad io-jet. The stronger radio-jet in NGC 1068 may have cleared a channel throu gh the NLR allowing some unattenuated nuclear continuum to illuminate part of the coronal line region.