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.

Allen, M. G., & Groves, B. A., & Dopita, M. A., & Sutherl, R. S., & Kewley, L. J.
2008, ApJS, 178, 20
Antonucci, R. R. J., & Miller, J. S. 1985, ApJ, 297, 621
Antonucci, R. R. J. 1993, ARA&A, 31, 473
Baldwin, J. A., Phillips, M. M., & Terlevich, R. 1981, Pub. A. S. P., 93, 5
Balgoh, M. L., Morris, S. L., Yee, H. K. C., Carlberg, R. G., & Ellingson, E. 1999,
ApJ, 527, 54
Bruzual, A. G. 1983, ApJ, 273, 105
Bruzual, G., & Charlot, S. 2003, MNRAS, 344, 1000
Bian, W., & Gu, Q. 2007, ApJ, 657, 159
Cid Fernandes, R., Mateus, A., Sodr´e, L., Stasi´nska, G., & Gomes, J. M. 2005,
MNRAS, 358, 363
de Grijp, M. H. K., Keel, W. C., Miley, G. K., Goudfrooij, P., & Lub, J. 1992,
A&AS, 96, 389
Deluit, S. J. 2004, A&A, 415, 39
Dudik, R. P., Satyapal, S., & Marcu, D. 2009, ApJ, 691, 1501
Ferland, G. J., Korista, K. T., Verner, D. A., Ferguson, J. W., Kingdon, J. B., &
Verner, E. M. 1998, PASP, 110, 761
Frogel, J. A., Gillett, F. C., Terndrup, D. M., & Vader, J. P. 1989, ApJ, 343, 672
Gu, Q., & Huang, J. 2002, ApJ, 579, 205
Gu, Q., Melnick, J., Cid Fernandes, R., Kunth, D., Terlevich, E., & Terlevich, R.
2006, MNRAS, 366, 480
Gonzalez Delgado, R. M., Heckman, T., & Leitherer, C. 2001, ApJ, 546, 845
Heckman, T. M. 1980, A&A, 87,152
Heisler, C. A., Lumsden, S. L., & Bailey, J. A. 1997, Nature, 385, 700
Ho, L. C., Filippenko, A. V., & Sargent, W. L. W. 1997, ApJS, 112, 315
Kaspi, S., Maoz, D., Netzer, H., Peterson, B. M., Vestergaard, M., & Jannuzi, B.
T. 2005, ApJ, 629, 61
Kauffmann, G., et al. 2003, MNRAS, 346, 1055
Khachikian, E. Y., & Weedman, D. W. 1974, ApJ, 192, 581
Komossa, S., & Xu, D. 2007, ApJ, 667, L33
Koski, A. T. 1978, ApJ, 223, 56
Kellermann, K. I., Sramek, R., Schmidt, M., Shaffer, D. B., & Green, R. 1989, AJ,
98, 4
Kewley, L. J., Dopita, M. A., Sutherland, R. S., Heisler, C. A., & Trevena, J. 2001,
ApJ, 556, 121
Kewley, L. J., Heisler, C. A., & Dopita, M. A. 2001, ApJS, 132, 37
Kewley, L. J., Groves, G., Kauffmann, G., & Heckman, T. 2006, MNRAS, 372, 961
Matteucci, F., & Padovani, P., 1993, ApJ, 419, 485
Moran, E. C., Barth, A. J., Kay, L. E., & Filippenko, A. V. 2000, ApJ, 540, L73
Moran, E. C., Kay, L. E., Davis, M., Filippenko, A. V., & Barth, A. J. 2001, ApJ,
556, L75
Nicastro, F., Martocchia, A., & Matt, G. 2003, ApJ, 589, L13
Osterbrock, D. E., & De Robertis, M. M. 1985, Pub. A. S. P., 97, 1129
Osterbrock, D. E. 1989, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei,
University Science Books, ISBN 0-935702-22-9
Osterbrock, D. E., & Martel A. 1993, ApJ, 414, 552
Panessa, F., & Bassani, L. 2002, A&A, 394,435
Peterson, B. M., 2006, LNP 693, 77
Peterson, B. M., 1997, An Introduction to Active Galactic Nuclei, Cambridge University
Press, ISBN 0-521-47348-9
P´erez, E., Manchado, A., Garcia-Lario, P., & Pottasch, S. R. 1989, A&A, 227, 407
Sandage, A. 1965, ApJ, 141, 1560
Seyfert, C. K. 1943, ApJ, 97, 28
Shlosman, I., Frank, J., & Begelman, M. C. 1989, Nature, 338, 45
Shu, X. W., Wang, J. X., Jiang, P., Fan, L. L., & Wang, T. G. 2007, ApJ, 657, 167
Shuder, J. M., & Osterbrock, D. E. 1981, ApJ, 250, 55
Sturm, E. et al. 2006, ApJ, 653, L13
Storchi-Bergmann, T., & Pastoriza, M. G. 1990, PASP, 102, 1359
Strochi-Bergmann, T., Raimann, D., Bica, E. L. D., & Fraquelli, H. A. 2000, ApJ,
544, 747
Strochi-Bergmann, T., Gonzalez Delgado, R. M., Schmitt, H. R., Fernands, R. C.,
& Heckman, T. 2001, ApJ, 559, 147
Terlevich, R., Melnick, J., Masegosa, J., Moles, M., & Copetti, M. V. F. 1991,
A&AS, 91, 285
Tommasin, S. et al. 2012, ApJ, 753, 155
Tran, H. D. 1995, ApJ, 440, 565
Tran, H. D. 1995, ApJ, 440, 597
Tran, H. D. 2003, ApJ, 583, 632
Tran, H. D. 2010, ApJ, 711, 1174
Tran, H. D., Lyke, J. E., & Mader, J. A. 2011, ApJ, 726, L21
Vaceli, M. S., Viegas, S. M., Gruenwald, R., & de Souza, R. E. 1997, AJ, 114, 1345
Vader, J. P., Frogel, J. A., Terndrup, D. M., & Heisler C. A. 1993, AJ, 106, 1743
Veilleux, S., & Osterbrock, D. E. 1987, ApJS, 63, 295
Veilleux, S., Kim, D. -C., Sanders, D. B., Mazzarella, J. M., & Soifer, B. T. 1995,
ApJS, 98, 171
Worthey, G. 1994, ApJS, 95, 107
Wu, Y. Z., Zhang, E. P., Liang, Y. C., Zhang, C. M., & Zhao, Y. H. 2011, ApJ, 730,
121
Wu, Y. Z., Zhao, Y. H., & Meng, X. M. 2011, ApJS, 195, 17
Young, S., Hough, J. H., Efstathiou, A., Wills, B. J., Axon, D. J., Bailey, J. A., &
Ward, M. J. 1996a, MNRAS, 279, L72
Young, S., Hough, J. H., Efstathiou, A., Wills, B. J., Bailey, J. A., Ward, M. J. &
Axon, D. J. 1996b, MNRAS, 281, 1206
Yu, P-. C., & Hwang, C-. Y. 2005, ApJ, 631, 720
Yu, P-. C., & Hwang, C-. Y. 2011, AJ, 142, 14
Yu, P-. C., Huang, Kui-Yun, Hwang, C-. Y., & Youichi Ohyama 2013, ApJ, 768, 30
Zhang, E., & Wang, J. 2006, ApJ, 653, 137