Mid-infrared properties of low-metallicity blue compact dwarf galaxies from the Spitzer infrared spectrograph

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 mu m 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.