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The properties of narrow-line Seyfert 1 (NLS1) galaxies, the links andcorrelations between them, and the physics behind them, are still not wellunderstood. Apart from accretion rates and black hole masses, density andoutflows were speculated to be among the main drivers of the NLS1 phenomenon.Here, we utilize the diagnostic power of the [SII]6716,6731 intensity ratio tomeasure the density of the NLR systematically and homogeneously for a largesample of NLS1 galaxies, and we perform a comparison with a sample ofbroad-line type 1 AGN. We report the discovery of a zone of avoidance indensity in the sense that AGN with broad lines (FWHM_Hbeta > 2000 km/s) avoidlow densities, while NLS1 galaxies show a wider distribution in the NLRdensity, including a significant number of objects with low densities. Acorrelation analysis further shows that the Eddington ratio L/L_Eddanti-correlates with density. We investigate a number of different models forthe zone of avoidance in density. Supersolar metallicities and temperatureeffects, a strong starburst contribution in NLS1 galaxies, different NLRextents and selective obscuration are considered unlikely. Possible differencesin the fraction of matter-bounded clouds and differences in the interstellarmedia of the host galaxies of NLS1 galaxies and broad-line Seyfert 1 (BLS1)galaxies can only be tested further with future observations. We tentativelyfavor the effects of winds/outflows, stronger in NLS1 galaxies than in BLS1galaxies, to explain the observations.

The narrow-line region of narrow-line and broad-line type 1 AGN I. A zone of avoidance in density