Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...312..365b&link_type=abstract
Astronomy and Astrophysics, v.312, p.365-379
Astronomy and Astrophysics
Astrophysics
137
Galaxies: Active, Galaxies: Seyfert, Line: Formation, Galaxies: Ism
Scientific paper
Recent reliable measurements of the weak lines of [OIII]λ4363, HeIIλ4686 and [NeV]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 HeII/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 HeII emission, and an ionization-bounded (IB) component emitting low-to-intermediate excitation lines. A new sequence of photoionization calculations is thereby obtained by varying A_M/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 A_M/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 HeII/Hβ accessible to models without requiring changes in the spectral index of the ionizing source, which we fix at α=-1.3. The main success of the model is that it provides a natural explanation for the newly found correlations between both the [OII]/[NeV] and [OII]/[OIII] ratio and the HeII/Hβ ratio. On the other hand, the standard U-sequence cannot account for such correlations between the gaseous excitation and the HeII/Hβ ratio. Furthermore, the so-called `temperature problem' is solved with the A_M/I_-sequence provided the thickness and the ionization parameter (U_MB_>0.03) of the MB component are appropriately selected. Finally, the A_M/I_-sequence produces much stronger high excitation lines of [NeV] and CIVλλ1549 than the U-sequence, in accordance with observations. The A_M/I_-sequence predicts a temperature difference of 5000K between the T_OIII_ and T_NII_ temperatures while for the U-sequence this difference is less than 1000K. 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 HeII/Hβ<0.01 while the U-sequence is charaterized by a value >0.1.
Binette Luc
Storchi-Bergmann Thaisa
Wilson Andrew S.
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