Interpreting the Ionization Sequence in Star-Forming Galaxy Emission-Line Spectra

TL;DR

This paper models the ionization sequence in star-forming galaxy emission spectra using a cloud model, revealing that ionizing flux distribution, alongside metallicity, primarily drives the observed ionization levels.

Contribution

It introduces a new interpretation of the ionization sequence as driven mainly by ionizing flux distribution, expanding beyond metallicity as the sole controlling parameter.

Findings

The ionization sequence can be explained by variations in ionizing flux distribution.

Metallicity remains a necessary factor but is not the sole determinant.

Galaxies with extreme ionization levels require additional physical parameters.

Abstract

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.