Tracing the [FeII]/[NeII] ratio and its relationship with other ISM indicators within star forming dwarf galaxies: a Spitzer IRS archival study

TL;DR

This study analyzes archival Spitzer data of 41 starburst dwarf galaxies to explore how the [FeII]/[NeII] emission line ratio relates to gas density and metallicity, revealing new insights into ISM conditions and shock contributions.

Contribution

It establishes the first correlation between [FeII]/[NeII] ratio and electron gas density, and clarifies the influence of metallicity on iron emission in star-forming dwarf galaxies.

Findings

[FeII]/[NeII] ratio decreases with increasing gas density.

Strong correlation between gas density and PAH peak to continuum.

Metallicity influences the origin of [FeII] emission, from shocks to dense PDR regions.

Abstract

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 micron ratio and the electron gas density as traced by the 18.7/33.4 micron [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 HII 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.