PHOTOIONIZATION OF VERY HIGH-EXCITATION GAS IN THE CIRCINUS GALAXY AND OTHER ACTIVE GALACTIC NUCLEI

Recent measurements of the Circinus galaxy with ISO by Moorwood et al. (1996) have confirmed the extremely high excitation of the emission-l ine spectrum in this active galaxy. Moorwood et al. modelled the spect rum in terms of photoionization of gas by a hard X-ray continuum plus a UV bump near 70 eV. We present new photoionization calculations whic h differ markedly in geometry and in the adopted spectrum of the ioniz ing radiation. In our model, all the zones of high excitation coexist within the internal structure of matter-bounded clouds of 1 pc thickne ss with a total cross section of 165 pc(2). A strong UV bump is not re quired to explain the line ratios in this picture. Our calculations co nsider the effects of radiation pressure exerted by photoelectric abso rption which become very important at the high ionization parameters r equired by the line ratios. We propose that the radiation pressure gen erates a strong density gradient within the photoionized structure and is responsible for accelerating the matter-bounded gas, explaining th e systematic blueshift observed for the coronal lines.