2023/09/02 by XueGuang, Zhang
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2309.00852
In the manuscript, we check properties of electron densities ne traced by flux ratio Rsii of [S~\sc ii]λ6716Å~ to [S~\sc ii]λ6731Å~ 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 ne 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 Rsii (higher ne) in NLRs can be well confirmed in Type-1 AGN than in Type-2 AGN, with confidence level higher than 5σ, even after considering necessary effects including effects of electron temperatures traced by [O~\sc iii]λ4364,4959,5007Å~ on estimating ne in NLRs. Two probable methods are proposed to explain the higher ne in NLRs in Type-1 AGN. First, the higher ne 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 ne in NLRs of Type-2 AGN than HII galaxies. Second, the lower ne in NLRs of Type-2 AGN could be explained by stronger star-forming contributions in Type-2 AGN, considering lower ne in HII regions. The results provide interesting challenges to the commonly and widely accepted Unified Model of AGN.