Ionization Mechanisms of HBLR and Non-HBLR Seyfert 2 Galaxies

TL;DR

This study compares ionization mechanisms in HBLR and non-HBLR Seyfert 2 galaxies, revealing nitrogen overabundance in non-HBLR types likely due to stellar evolution, suggesting an evolutionary link.

Contribution

It demonstrates that nitrogen overabundance in non-HBLR Seyfert 2 galaxies cannot be explained by standard ionization models and proposes stellar evolution as the cause.

Findings

Non-HBLR Seyfert 2 galaxies have higher [N II]/H_alpha ratios.

Standard ionization models cannot explain nitrogen overabundance.

Stellar evolution may cause nitrogen overabundance, linking HBLR and non-HBLR Seyfert 2 galaxies.

Abstract

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.