Dusty, Radiation Pressure Dominated Photoionization: The Solution to the   Narrow Line Region Problem
  Seyfert narrow line regions (NLR) have emission line ratios which are remarkably uniform, displaying only ~0.5 dex variation between different galaxies. Existing models have been unable to explain these observations without the introduction of ad hoc assumptions, geometrical restrictions or new parameters. Here we introduce a new model: dusty radiation pressure dominated photoionization, which provides a natural self-regulating characteristic leading to an invariance of the spectrum over a very wide range (>100) of ionization parameter. The dusty model is able to reproduce both the range and the absolute value of the observational line ratios not only in the standard optical diagnostic diagrams but also in UV diagnostic plots, providing an explanation to the problem in NLR observations. 
Brent Groves, Michael Dopita, Ralph Sutherland (RSAA, Australian   National University)