AGN Emission Lines
•Hagai Netzer’s section in Saas-Fee Advanced Courses 20, 1990, available online:
–http://nedwww.ipac.caltech.edu/level5/March02/Netzer/Netzer_contents.html 
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•Classic textbook on photoionization is AGN2 by Don Osterbrock, popular public tool is CLOUDY by Gary Ferland (http://thunder.pa.uky.edu/cloudy/ ).  Section 9.1.2 in Combes et al.
Basically, treat ionization state, heating/cooling balance, and relate emission line ratios to metallicity, density, ionizing continuum, etc.  Note “LOC” models (Baldwin et al. 1996).