  High ionization star forming (SF) galaxies are easily identified with strong
emission line techniques such as the BPT diagram, and form an obvious
ionization sequence on such diagrams. We use a locally optimally emitting cloud
model to fit emission line ratios that constrain the excitation mechanism,
spectral energy distribution, abundances and physical conditions along the
star-formation ionization sequence. Our analysis takes advantage of the
identification of a sample of pure star-forming galaxies, to define the
ionization sequence, via mean field independent component analysis. Previous
work has suggested that the major parameter controlling the ionization level in
SF galaxies is the metallicity. Here we show that the observed SF- sequence
could alternatively be interpreted primarily as a sequence in the distribution
of the ionizing flux incident on gas spread throughout a galaxy. Metallicity
variations remain necessary to model the SF-sequence, however, our best models
indicate that galaxies with the highest and lowest observed ionization levels
(outside the range -0.37 < log [O III]/H\b{eta} < -0.09) require the variation
of an additional physical parameter other than metallicity, which we determine
to be the distribution of ionizing flux in the galaxy.
