An improved Seyfert-low ionization nuclear emission-line region classification line in the [N II] BPT diagram

School of Physical Science and Technology, Guangxi University, No. 100, Daxue East Road, Nanning, 530004, P.R. China

^⋆ Corresponding author: xgzhang@gxu.edu.cn

Received: 9 March 2025
Accepted: 23 May 2025

Abstract

Context. In this paper, we propose an improved Seyfert-low ionization nuclear emission-line region (LINER) classification line (=S-L line) in the [N II] BPT diagram, based on a sample of 47 968 low-redshift, narrow-emission-line galaxies from SDSS DR16, motivated by different S-L lines reported in the [N II] BPT diagram through different methods. We first applied the method successfully applied in the [S II] and [O I] BPT diagrams, however, the method does not lead to an accepted S-L line in the [N II] BPT diagram. Meanwhile, the S-L lines previously proposed in the [N II] BPT diagram are different from each other.

Aims. We aim to check which proposed S-L line is better or to determine an improved one in the [N II] BPT diagram by a new method.

Methods. We visualized the Seyferts and LINERs that had already been classified in the [S II] and/or [O I] BPT diagrams in the [N II] BPT diagram. The intersection boundary between the two contour maps was then considered as the S-L line in the [N II] BPT diagram.

Results. Rather than the previously proposed S-L lines, the new S-L line can lead to more efficient and harmonious classifications of Seyferts and LINERs, especially in the composite galaxy region, in the [N II] BPT diagram. Furthermore, based on the discussed S-L lines, the number ratio of Type-2 Seyferts to Type-2 LINERs differs significantly from that of Type-1 Seyferts to Type-1 LINERs in the [N II] BPT diagram. This suggests that about 90% of Type-2 LINERs are non-active galactic nucleus (non-AGN)-related objects, true Type-2 AGNs, or objects that exhibit both Seyfert and LINER characteristics.