2024ApJ...960..108Z

In the manuscript, we check properties of electron densities n_e traced by flux ratio R_sii of [S II]λ6716 Å to [S II]λ6731 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 5σ, even after considering the necessary effects including effects of electron temperatures traced by [O III]λ4364,4959,5007 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.