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Objects with weak [NII] lines

  Figure [*] shows a small number of objects which have very weak Nii lines for Seyfert 2 galaxies; their Oi lines are however normal for this class of objects.

The first photoionization models invoked to explain the narrow emission lines in AGNs assumed a single density cloud. However, new observations quickly suggested the presence of several emitting clouds, ruling out single component models. Most of the multicloud models first studied were such that the emitting gas, as a whole, was ionization-bounded and thus the Heii$\lambda$4686 line intensity relative to $\rm H\beta$ was determined by the hardness of the ionizing spectrum. In these models, the extreme values reached by the $\lambda$4686/H$\beta$ ratio are not well reproduced. A number of objects have $\lambda$4686/H$\beta$ of the order of 0.2 or more; such high values cannot be accounted for unless the line emitting clouds are matter-bounded (Stasinska 1984). On the basis of a weak trend for the low excitation lines to become weaker as $\lambda$4686/H$\beta$ gets larger, Viegas-Aldrovandi (1988) and Viegas & Prieto (1992) argued in favor of a model in which matter-bounded clouds are present; indeed, if the gas is not optically thick to all the ionizing continuum (i.e., is matter bounded), the H+ emitting volume is smaller, but the He++ volume is not, leading to a higher $\lambda$4686/H$\beta$ line ratio. Moreover, Viegas-Aldrovandi & Gruenwald (1988) and Rodriguez-Ardila et al. (1998) showed that, for most AGNs, the observed low-excitation lines are better explained by matter-bounded models with about 50% of the $\rm H\beta$ luminosity produced in ionization-bounded clouds.

Storchi-Bergmann et al. (1996a) have obtained long-slit spectra of five active galaxies showing extended high excitation lines. At some positions, two of the objects (PKS 0349-27 and PKS 0634-20) show quite peculiar line ratios, with a strong Heii$\lambda$4686 line ($\lambda$4686/H$\beta$ > 0.3) and weak Nii lines (that is, $\lambda$6583/H$\alpha$ < 0.3). In fact, there seems to be a correlation between $\lambda$6583/H$\alpha$ and $\lambda$4686/H$\beta$, weak Nii lines being associated with strong Heii emission, suggesting that very small $\lambda$6583/H$\alpha$ ratios (as observed in the two above mentioned radiogalaxies) are not necessarily a signature of star-formation, but a natural consequence of having a region dominated by matter-bounded clouds (Binette et al. 1996, 1997). However, in the extranuclear regions of PKS 0349-278 in which strong Heii$\lambda$4686 and weak Nii$\lambda$6583 lines are observed, the Oi $\lambda$6300 line is also reduced ($\lambda$6300/H$\alpha$ $\sim$ 0.05), which is a natural consequence of the model (Viegas-Aldrovandi 1988), while in our sample of weak Nii$\lambda$6583 galaxies, we verify that the Oi $\lambda$6300 line is not weakened in most of the objects.

In Table [*] we give the list of known AGNs with relatively weak Nii lines ($\lambda$6583/H$\alpha$ < 0.45) with published values of the $\lambda$4686/H$\beta$ and $\lambda$6300/H$\alpha$ ratios. Three objects in this table (UM 85, MS 04124-0802 and Mark 699) have both weak Nii lines ($\lambda$6583/H$\alpha$ < 0.20) and a strong Heii line ($\lambda$4686/ $\rm H\beta$ > 0.30). In the last two, the Oi lines are also relatively weak ($\lambda$6300/H$\alpha$ $\le$ 0.05); these two objects could be dominated by matter-bounded clouds. Alternatively, in the other objects, the weakness of the Nii lines could be due to a selective under-abundance of nitrogen. For a photoionized single cloud model with $U \sim$ 10-2.5, Ferland & Netzer (1983) predicted $\lambda$6583/H$\alpha$ $\sim$ 1.0 for solar nitrogen abundances and $\sim$ 0.3 for nitrogen abundances $\sim$ 0.3 solar.

Table:  Known AGNs with weak Nii lines.
Name Short $\lambda$6583$\:$ $\lambda$4686$\:$ $\lambda$6300$\:$
  position $\rm H\alpha$ $\rm H\beta$ $\rm H\alpha$
UM 85 0104+06 0.19 0.37 0.09
MS 04124-0802 0412-08 0.10 0.35 0.05
IRAS 04210+0400 0421+04 0.35 0.21 0.13
3C 184.1 0734+80 0.22 0.26 0.07
IRAS 11058-1131 1105-11 0.38 0.17 0.05
SBS 1136+594 1136+59 0.10 0.18 0.11
Mark 477 1439+53 0.36 0.13 0.17
Mark 699 1622+41 0.20 0.34 0.03

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Next: Seyfert 2s and Liners Up: Discussion Previous: Excitations and abundances in