2008ApJ...681.1205O

Archival Spitzer observations of 41 starburst galaxies that span a wide range in metallicity reveal for the first time a correlation between the [Fe II]/[Ne II] 26.0 µm/12.8 µm ratio and the electron gas density as traced by the 18.7 µm/33.4 µm [S III] ratio, with the [Fe II]/[Ne II] ratio decreasing with increasing gas density. The correlations of the [Fe II]/[Ne II] ratio, the PAH peak-to-continuum strength, and 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 that the driver of the observed [Fe II ]/[Ne II] ratios is metallicity. The majority of [Fe II] emission in low-metallicity galaxies may be shock-derived, while at high metallicity, the [Fe II] emission may be instead dominated by contributions from H II and in particular from dense PDR regions. However, the observed [Fe II]/[Ne II] 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.