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

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