Testing the evolutionary sequence between hidden broad-line region (HBLR) and non-Hblr Seyfert 2 galaxies with the 4000 å break strengths

We compare the 4000 Å break (D_n (4000)) strength in the central kpc of hidden broad-line region (HBLR) and non-HBLR Seyfert 2 galaxies to investigate the origin of these galaxies. Our results show that the D _n (4000) strengths in the nuclear regions of the non-HBLR Seyfert 2 galaxies are larger than those in the HBLR galaxies. We also show that the D_n (4000) strength is not related to the morphology of host galaxies. These results imply that the non-HBLR Seyfert 2 galaxies have an older stellar population in nuclear regions than the HBLR galaxies. This suggests that an evolutionary connection might exist between non-HBLR and HBLR Seyfert 2 galaxies. 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 implications of our results in the hydrogen column density distribution of the non-HBLR Seyfert 2 galaxies.