Excitation and temperature of extended gas in active galaxies. II Photoionization models with matter-bounded clouds

Abstract

Recent reliable measurements of the weak lines of [O III]λ4363, He IIλ4686 and [Ne V]λ3426 seen in emission in extended and nuclear regions of AGN (Storchi-Bergmann et al. 1996) are used to discriminate between different types of photoionization models. The sample comprises Seyferts and Narrow Line Radio Galaxies. A well known problem, the scatter and the extreme values of He II/Hβ encountered in the nuclei and extended regions, is solved in a similar fashion to Viegas & Prieto (1992) by considering two populations of ionized clouds: a matter-bounded (MB) component responsible for most of the He II emission, and an ionization-bounded (IB) component emitting low-to-intermediate excitation lines. A new sequence of photoionization calculations is thereby obtained by varying AM/I, which is defined as the solid angle ratio occupied by the MB component relative to the IB component. In various line ratio diagrams, the AM/I-sequence is compared to the traditional single component U-sequence in which one arbitrarily varies the relative intensity of the ionizing source. An original aspect of the new calculations is that the IB clouds are photoionized exclusively by the radiation which has filtered ('leaked') through the MB component. This situation could arise if, for example, the MB component is the low density 'halo' of a cloud, and the IB component the high density core. Such a filtered ionizing spectrum enlarges the range in He II/Hβ accessible to models without requiring changes in the spectral index of the ionizing source, which we fix at =–α1.3. The main success of the model is that it provides a natural explanation for the newly found correlations between both the [O II]/[Ne V] and [O II]/[0 III] ratio and the He II/Hβ ratio. On the other hand, the standard U-sequence cannot account for such correlations between the gaseous excitation and the He II/Hβ ratio. Furthermore, the so-called 'temperature problem' is solved with the AM/I-sequence provided the thickness and the ionization parameter (UMB>~(0.03) of the MB component are appropriately selected. Finally, the AM/I-sequence produces much stronger high excitation lines of [Ne V] and C IV λλ1549 than the U-sequence, in accordance with observations. The AM/I-sequence predicts a temperature difference of 5 000 K between the T OIII and T NII temperatures while for the U-sequence this difference is less than 1 000 K. To the extent that 'cooling flows' and LINERs share the same excitation mechanism as our IB component (i.e. photoionization by an absorbed ionizing continuum), we predict a value of He II/Hβ<0.01 while the U-sequence is charaterized by a value >0.1.