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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 issignificantly higher than that of the HBLR Seyfert 2 galaxies while other lineratios, such as [O III]/H_beta and [O I]/H_beta are similar. To probe theorigin of this difference, we explore theoretical results of differentionization models, such as photoionization, starburst, and shock models. Wefind that none of these models can explain the high [N II]6583/H_alpha ratio ofthe non-HBLR Seyfert 2 galaxies with solar abundance; the high [NII]6583/H_alpha must be reproduced from enhanced nitrogen abundance. Sincenitrogen overabundance can be achieved from the dredge-up of red supergiants inthe post-main-sequence stage, we suggest that the observed nitrogenoverabundance of the non-HBLR Seyfert 2 might be caused by stellar evolution,and there could be an evolutionary connection between the HBLR and non-HBLRSeyfert 2 galaxies.

Ionization Mechanisms of HBLR and Non-HBLR Seyfert 2 Galaxies