The Narrow Emission Lines of Seyfert 1 Galaxies: Comparisons with a Large SDSS Sample

TL;DR

This study analyzes SDSS spectra of Seyfert galaxies, revealing that narrow-line regions vary with Seyfert type and star formation, challenging simple unification models.

Contribution

It provides a detailed comparison of narrow-line regions across Seyfert types and introduces a model explaining their composite spectra with star formation contributions.

Findings

Most NL of strong Sy 1.0 and Sy 1.2 are similar to Sy2s.

Only 25-30% of Sy1.8 and Sy1.9 nuclei are in the pure Sy2 region.

NL in Sy1.9 mainly originate from HII regions.

Abstract

We analyzed a large sample of SDSS spectra of Seyfert galaxies, subdividing Seyfert 1s based on their narrow-to-broad Halpha components. Comparing their narrow-lines (NL) to Seyfert2s in line-ratio diagrams, most of the NL of strong Sy 1.0 and Sy 1.2s (with dominant broad lines) are the same as those of pure Sy2s. In contrast, only 25-30 percent of the Sy1.8 and Sy1.9 nuclei (with weak broad lines) are located in the pure Sy2 region, with the rest falling in the composite-star-forming region. We explain these Seyfert-plus-star-formation spectra with a simple model. It shows that 85 percent of NL in Sy1.9 are from HII-regions, while 88 percent of the NL in Sy 1.0 arise from the same NLR as in pure Sy2. About 6 percent of the strong and weak Sy1's have NL dominated by LINER emission, while 15 percent of intermediate Seyferts (Sy 1.5 and Sy 1.6) do. To confirm this Seyfert 1 AGN plus star formation combination, we used stellar absorption-lines to compare their stellar populations. Their Hdelta strengths show that LINERs, pure Sy2s, and also the broad-line dominated Sy1s have old stellar populations. The weak Sy 1s show stronger Hdelta absorption, indicating larger proportions of young stars. About one third of the u band light in Sy1.0 and 1.2 is blended Balmer lines and continuum from the BLR. The NL gas reddening increases as the BLR strength decreases, from Sy1.0 (0.13 mag), to Sy1.9 (0.40 mag), to Sy2s and LINERs both with 0.50 mag. Our data do not support the simplest version of Seyfert 1 and 2 unification, where both AGN classes have identical NL.