Tracing the [Fe ii ]/[Ne ii ] Ratio and Its Relationship with Other ISM Indicators within Star‐forming Dwarf Galaxies: A Spitzer IRS Archival Study

Abstract

Archival Spitzer observations of 41 starburst galaxies that span a widerange in metallicity reveal for the first time a correlation between the[Fe II]/[Ne II] 26.0 {μ}m/12.8 {μ}m ratio and the electron gas densityas traced by the 18.7 {μ}m/33.4 {μ}m [S III] ratio, with the [FeII]/[Ne II] ratio decreasing with increasing gas density. Thecorrelations of the [Fe II]/[Ne II] ratio, the PAH peak-to-continuumstrength, and metallicity found in an earlier paper were confirmed for alarger sample. We also find a strong correlation between the gas densityand the PAH peak-to-continuum strength. Using shock and photoionizationmodels, we see that the driver of the observed [Fe II ]/[Ne II] ratiosis metallicity. The majority of [Fe II] emission in low-metallicitygalaxies may be shock-derived, while at high metallicity, the [Fe II]emission may be instead dominated by contributions from H II and inparticular 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, aresult that would have profound implications for the use of ironemission lines as unambiguous tracers of shocks.