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

Abstract

We present photoionization models of the optical and IR coronal line spectrum in NGC1068 and the Circinus galaxy. The line fluxes have been calculated using (a) a non-thermal (nuclear) continuum source and (b) the non-thermal continuum plus a UV bump due to a stellar cluster. We take into account the 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 calculated coronal line ratios are in a good agreement with those observed in NGC1068 for a model in which about 40% of the line emission comes from gas illuminated by unattenuated, non-thermal continuum, and about 60% from gas illuminated by attenuated, non-thermal continuum. The electron density of the coronal line emitting gas in NGC1068 is found to be $10^4 cm^{-3}$. In the Circinus galaxy the coronal line emission comes from gas with electron density $10^3 cm^{-3}$ 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 NGC1068, consistent with the higher observed excitation state of the line emitting gas in that galaxy. A possible geometry of the coronal line region of both galaxies is discussed, in which the distribution of the attenuating gas may be strongly affected by the relative strength of the nuclear radio-jet. The stronger radio-jet in NGC1068 may have cleared a channel through the NLR allowing some unattenuated nuclear continuum to illuminate part of the coronal line region.Comment: Accepted for publication in Astronomy and Astrophysics (main journal