Ionization Mechanisms of HBLR and Non-HBLR Seyfert 2 Galaxies
  We investigate the ionization mechanisms for hidden broad-line region (HBLR)
and non-HBLR Seyfert 2 galaxies by comparing some optical emission line ratios.
We note that the [N II]6583/H_alpha ratio of the non-HBLR Seyfert 2 galaxies is
significantly higher than that of the HBLR Seyfert 2 galaxies while other line
ratios, such as [O III]/H_beta and [O I]/H_beta are similar. To probe the
origin of this difference, we explore theoretical results of different
ionization models, such as photoionization, starburst, and shock models. We
find that none of these models can explain the high [N II]6583/H_alpha ratio of
the non-HBLR Seyfert 2 galaxies with solar abundance; the high [N
II]6583/H_alpha must be reproduced from enhanced nitrogen abundance. Since
nitrogen overabundance can be achieved from the dredge-up of red supergiants in
the post-main-sequence stage, we suggest that the observed nitrogen
overabundance of the non-HBLR Seyfert 2 might be caused by stellar evolution,
and there could be an evolutionary connection between the HBLR and non-HBLR
Seyfert 2 galaxies.
