Modelling narrow-line regions of active galaxies in the Sloan Digital Sky Survey - I.Sample selection and physical conditions

We systematically determine the electron density and electron temperature of active galaxies and star-forming galaxies using spectroscopy from the SDSS DR7, while mainly focusing on the NLRs. Density and temperature are determined through the I[S II] 6716/6731 and I[O III] 5007/4363 ratios, respectively, in our [O III] 4363 emission sample of 15 019 galaxies. We find two sequences and the typical range of density in the NLRs of AGNs is 100-1000 /cm3. The temperatures in the NLRs range from 10 000 to 20 000 K for Seyferts, and the ranges were even higher and wider for LINERs and composites. We also propose that Y_LINER ~ Y_Seyfert > Y_composite > Y_star-forming, where Y is the characteristic present-day star-formation time-scale. While in the AGN case, we find several strong lines of evidence suggest that some supplementary energy source(s) should be responsible for high ionization potential.