Are there higher electron densities in narrow emission line regions of Type-1 AGN than Type-2 AGN?

Abstract

In the manuscript, we check properties of electron densities $n_e$ traced by flux ratio $R_{sii}$ of [S~{\sc ii}]$\lambda6716$\AA~ to [S~{\sc ii}]$\lambda6731$\AA~ in narrow emission line regions (NLRs) between Type-1 AGN and Type-2 AGN in SDSS DR12. Under the framework of Unified Model considering kpc-scale structures, similar $n_e$ in NLRs should be expected between Type-1 AGN and Type-2 AGN. Based on reliable measurements of [S~{\sc ii}] doublet with measured parameters at least five 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 confidence level higher than 5$\sigma$, even after considering necessary effects including effects of electron temperatures traced by [O~{\sc iii}]$\lambda4364,4959,5007$\AA~ 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 HII 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 HII regions. The results provide interesting challenges to the commonly and widely accepted Unified Model of AGN.