Are There Higher Electron Densities in Narrow Emission Line Regions of Type-1 AGNs than in Type-2 AGNs?

Abstract

In the manuscript, we check properties of electron densities n _e traced by flux ratio R _sii of [S ii ] λ 6716 Å to [S ii ] λ 6731 Å in narrow emission line regions (NLRs) between Type-1 active galactic nucleus (AGN) and Type-2 AGN in SDSS Data Release 12 (DR12). Under the framework of unified model considering kiloparsec-scale structures, similar n _e in NLRs should be expected between Type-1 AGN and Type-2 AGN. Based on reliable measurements of the [S ii ] doublet with measured parameters at least 5 times larger than corresponding uncertainties, there are 6039 Type-1 AGN and 8725 Type-2 AGN (excluding the Type-2 LINERs and the composite galaxies) collected from SDSS DR12. Then, lower R _sii (higher n _e ) in NLRs can be well confirmed in Type-1 AGN than in Type-2 AGN, with the confidence level higher than 5 σ , even after considering the necessary effects including effects of electron temperatures traced by [O iii ] λ 4364,4959,5007 Å on estimating n _e in NLRs. Two probable methods are proposed to explain the higher n _e in NLRs in Type-1 AGN. First, the higher n _e in NLRs of Type-1 AGN could indicate longer time durations of AGN activities in Type-1 AGN than in Type-2 AGN, if AGN activities triggering galactic-scale outflows leading to more electrons injecting into NLRs were accepted to explain the higher n _e in NLRs of Type-2 AGN than H ii galaxies. Second, the lower n _e in NLRs of Type-2 AGN could be explained by stronger star-forming contributions in Type-2 AGN, considering lower n _e in H ii regions. The results provide interesting challenges to the commonly and widely accepted unified model of AGN.

Keywords

• Active galaxies
• Active galactic nuclei
• Emission line galaxies
• Seyfert galaxies