On the basis of the previous results we conclude that the data can only be reproduced if we vary the abundances of nitrogen and sulfur alone, from 0.5 to 5 times the solar value; it is not possible to reproduce them by varying the abundance of the heavy elements all together. As these results depend on the models used, we have also checked the grid of photoionization models of Stasinska (1984b) and have not found any significant difference for the models with similar parameters to those used in this work.
The diagrams [N II] / H
x [S
II] / H show some correlation between
these two ratios. A linear regression to the data points gives a
correlation coefficient of 0.64. Should this mean that the nitrogen
and sulfur abundances are correlated? The results of the previous
section indicate that the answer is yes, because, as was shown above,
a variation of U, gas density, and abundance of all the elements in
the models cannot reproduce the data in this diagram. We also point
out that it is not possible to cover the data by varying the abundance
of only N or S; It is necessary to vary the abundance of both elements
together. A better coverage of the data points can be obtained if we
consider the coupled effect of a varying density, ionization parameter
U, and N and S abundances, as shown in Figure
5. In this figure we
have considered two typical values for U: 10-3 and
10-3.5, densities in
the range 103-105, cm-3, and N and S
abundances 0.5 times solar, solar,
and two and three times solar. This figure shows that we can
successfully reproduce the data, although the effect of a varying U
intermixes somewhat with that of varying N and S abundances. It can
also be seen that the broadness of the relation can be explained by
the presence of clouds of different densities in the gas.
|
Figure 5. Sample galaxies (dots) and photoionization models (lines) showing the coupled effect of a varying gas density (103-105 cm-3) and ionization parameter (10-3 < U < 10-3.5) for nitrogen and sulfur abundances 0.5, 1, 2, and 3 times solar. The abundances of the other elements are solar. |
In a previous work
(Storchi-Bergmann 1990)
it has been found a
dependence of the [N II] / H
ratio observed for this same sample of
Seyfert 2's and LINERs with the effective aperture (l) used in the
observations (square ratio of the area at the galaxy corresponding to
the observing slit), it was found that for l < 1 kpc most galaxies
present N overabundance in the nucleus. On the basis of the
correlation between [N II] / H
and [S II] / H, we have
searched for the same effect for sulfur.
Figure 6 shows the [S II] /
H ratio plotted
against effective aperture, where it can be seen that high [S II] /
H
values are preferentially found for l < 1 kpc. The histogram
quantifies this result showing that 60% of the subsample with l < 0.5
kpc present [S II] / H > 0.6
(solar value). This result shows that
indeed the same effect found previously for N is occurring with S:
There is a sulfur overabundance in most Seyfert 2 and LINER nuclei,
which is very localized, being restricted to a region with a typical
diameter of 500 pc around the nucleus.
|
Figure 6. The dependence ofthe [S II] /
H |
The above could indicate that high N and S abundances are somehow
related to the nuclear activity in these galaxies. A preliminary
calculation (Cid Fernandes and Dottori 1990) shows that a model nebula
photolonized by a cluster of very hot stars (WARMERS)
(Terlevich and Melnick
1985)
should soon be contaminated by a great amount of
nitrogen due to mass loss of the stars in the Wolf-Rayet N phase. This
is one possibility for explaining the high [N II] /
H ratios. In
another possibility in which the gas is enriched by winds from normal
nuclear stars, our results introduce important constraints in the
stellar evolution at the nuclei.