Tracing the [feii]/[neii] Ratio and Its Relationship with Other Ism Indicators within Star Forming Dwarf Galaxies: a Spitzer Irs Archival Study

Archival Spitzer observations of 41 starburst galaxies that span a wide range in metallicity reveal for the first time a correlation between the [FeII]/[NeII] 26.0/12.8 μm ratio and the electron gas density as traced by the 18.7/33.4 μm [SIII] ratio, with the [FeII]/[NeII] ratio decreasing with increasing gas density. The correlations of the [FeII]/[NeII] ratio, the PAH peak to continuum strength & metallicity found in an earlier paper were confirmed for a larger sample. We also find a strong correlation between the gas density and the PAH peak to continuum strength. Using shock and photoionization models, we see the driver of the observed [FeII]/[NeII] ratios is metallicity. The majority of [FeII] emission in low metallicity galaxies may be shock-derived, whilst at high metallicity, the [FeII] emission may be instead dominated by contributions from H ii and in particular from dense PDR regions. However, the observed [FeII]/[NeII] ratios may instead be following a metallicity-abundance relationship, with iron being less depleted onto grains in low metallicity galaxies a result that would have profound implications for the use of iron emission lines as unambiguous tracers of shocks. Subject headings: galaxies: starburst galaxies: stellar content ISM: lines and bands infrared: galaxies