When Photoionization Is Not Enough

It is well-known that photoionization is the major mechanism powering nebulae emission-lines. For about 30 years, numerical simulations using photoionization codes have been employed to analyze HII regions, planetary nebulae and active galactic nuclei. Although these models can reproduce most of the observed features, some remain unexplained. Two possible causes for the discrepancies between observations and model predictions are discussed in this paper: (a) the models are not realistic enough; (b) an additional energy source is needed. The discussion here is mainly based on the results of photoionization models for the NW knot of I Zw 18.