Mid‐Infrared Properties of Low‐Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared Spectrograph

Abstract

We present a Spitzer-based mid-infrared (MIR) study of a large sample of blue compact dwarfs (BCDs) using the Infrared Spectrograph (IRS), including the first MIR spectrum of I Zw 18, the archetype for the BCD class and among the most metal-poor galaxies known. We show the spectra of polycyclic aromatic hydrocarbon (PAH) emission in a low-metallicity environment. We find that the equivalent widths (EWs) of PAHs at 6.2, 7.7, 8.6, and 11.2 μm are generally weaker in BCDs than in typical starburst galaxies and that the fine-structure line ratio, [Ne III]/[Ne II], has a weak anticorrelation with the PAH EW. A much stronger anticorrelation is shown between the PAH EW and the product of the [Ne III]/[Ne II] ratio and the UV luminosity density divided by the metallicity. We conclude that the PAH EW in metal-poor high-excitation environments is determined by a combination of PAH formation and destruction effects.