Photoionization of very high excitation gas in circinus galaxy and other active galactic nuclei

Abstract

Recent measurements of the Circinus galaxy with ISO by Moorwood et al. (1996) have confirmed the extremely high excitation of the emission-line spectrum in this active galaxy. Moorwood et al. modelled the spectrum in terms of photoionization of gas by a hard X-ray continuum plus a UV bump near 70 eV. We present new photoionization calculations which differ markedly in geometry and in the adopted spectrum of the ionizing radiation. In our model, all the zones of high excitation coexist within the internal structure of matter-bounded clouds of 1 pc thickness with a total cross section of 165 pc2. A strong UV bump is not required to explain the line ratios in this picture. Our calculations consider the effects of radiation pressure exerted by photoelectric absorption which become very important at the high ionization parameters required by the line ratios. We propose that the radiation pressure generates a strong density gradient within the photoionized structure and is responsible for accelerating the matter-bounded gas, explaining the systematic blueshift observed for the coronal lines.