As has been previously shown by several authors (e.g.,
Ferland and Netzer 1983;
Stasinska 1984a;
Binette 1985),
as well as by ourselves in a previous work
(Storchi-Bergmann and
Pastoriza 1989),
the emission-line ratios of Seyfert 2's and LINERs can be successfully
reproduced by model nebulae photoionized by a power-law continuum. So
we have calculated the theoretical emission-line ratios using the
photoionization code CLOUDY
(Ferland 1990)
for a constant density
nebula photoionized by a power-law continuum of the form
*F*_{}
^{}. In
order to reproduce the observations we have varied the slope
"" of
the ionizing continuum, the gas density, and the abundance of the
elements, as described below. The typical total hydrogen column
densities are in the range 10^{20}-10^{21} cm^{-2}.

As shock ionization also produces an enhancement of the [N II] /
H ratio
(Dopita 1977),
we have considered, for comparison, the
integrated shock models of
Viegas-Aldrovandi (1988),
which take into
account the contribution of clouds of different velocities (100 < *v* <
600 km s^{-1}) according to the distribution law *f(v)*
*v*^{-b}.