統一模型 (unification model) 認為活躍星系核不同的譜線特徵，例如西佛一型與二型 (Seyfert 1 and 2 galaxies) 的差別，是由於環繞吸積盤的塵埃環型體 (dusty torus) 遮蔽了部分視線，導致我們在西佛一型可以觀測到可見光寬譜線與窄譜線，而在西佛二型只能觀測到窄譜線。之前在一些西佛二型星系觀測到偏極化寬譜線(Polarized hidden broad-line region HBLR Seyfert 2) 進一步支持了統一模型，然而在觀測上，還有超過50%的西佛二型星系並未觀測到偏極化寬譜線 (non-HBLR Seyfert 2)，這個觀測結果挑戰了統一模型。在此論文中，我們比較了可見光的光譜，我們發現在那些未觀測到偏極化寬譜線的西佛二型星系，其 [N II]/Hα 的比值高於觀測到偏極化寬譜線的西佛二型星系，為了進一步討論造成此現象的原因，我們分別用了三種不同的模型來重新產生光譜，我們發現要產生這麼高比值的 [N II]/Hα，必須要用比較高的氮豐度，由於氮在恆星演化後期有可能會被增強，這個結果暗示了這兩種西佛二型星系可能處在不同的演化階段。為了進一步比較此兩種西佛二型星系的核心星球年齡，我們比較了波長四千埃附近的吸收程度，我們發現那些未觀測到偏極化寬譜線的西佛二型星系，其核心星球的年齡老於觀測到偏極化寬譜線的西佛二型星系，最後我們提出了一個可能的西佛星系演化模型，來進一步解釋所觀測到的現象。 Based on the unification model, Seyfert 2 galaxies are suggested to be the same objects as Seyfert 1 galaxies, but viewed from a different direction only narrow emission lines are presented in Seyfert 2 spectra. Photons from broad line regions could be scattered by electrons and thus might be detectable by polarimetry in Seyfert 2 galaxies. However, previous studies showed that only about 40% - 45% of Seyfert 2 galaxies show polarized hidden broad line regions (HBLR). The reason of the non-detection of the polarized broad lines in some Seyfert 2 (non-HBLR Seyfert 2) is still under debate.In this thesis, we investigate the ionization mechanisms for HBLR and non-HBLR Seyfert 2 galaxies by comparing optical emission line ratios. We note that the [N II]/H_alpha ratio of the non-HBLR Seyfert 2 galaxies is significantly higher than that of the HBLR Seyfert 2 galaxies. 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 the high [N II]/H_alpha ratio must be reproduced from enhanced nitrogen abundance. Since nitrogen overabundance can be achieved from the dredge-up of red supergaints in the post-main-sequence stage, we suggest that the observed nitrogen overabundance of the non-HBLR Seyfert 2 galaxies might be caused by stellar evolution.To further test the evolutionary sequence of the HBLR and non-HBLR Seyfert 2 galaxies, we compare the 4000AA~break (Dn(4000)) strength in the central kpc of these Seyfert 2 galaxies. Our results show that the Dn(4000) strengths in the nuclear regions of the non-HBLR Seyfert 2 galaxies are larger than those in the HBLR galaxies. These results imply that the non-HBLR Seyfert 2 galaxies have an older stellar population in nuclear regions than the HBLR galaxies. Therefore, we propose a potential evolutionary scenario and a modified unification model for Seyfert galaxies. In this scheme, Seyfert 1 and HBLR Seyfert 2 galaxies evolve into unabsorbed and absorbed non-HBLR Seyfert 2 galaxies. We also discuss the implication of our results in the hydrogen column density distribution of the non-HBLR Seyfert 2 galaxies.
