The polycyclic aromatic hydrocarbon emission deficit in low-metallicity galaxies -: A Spitzer view

Archival observations of 18 starburst galaxies that span a wide range in metallicity reveal for the first time a correlation between the ratio of emission-line fluxes of [Fe II] at 26 mu m and [Ne II] at 12.8 mu m and the 7.7 mu m PAH strength, with the [Fe II]/[Ne II] flux ratio decreasing with increasing PAH strength. We also find a strong correlation between the [Fe II]/[Ne II] flux ratio and the host galaxy metallicity, with the flux ratio decreasing with increasing metallicity. Since [Fe II] emission has been linked primarily to supernova shocks, we attribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies to enhanced supernova activity. We consider this to be a dominant mechanism for PAH destruction, rather than grain destruction in photoionized regions surrounding young massive stars. We also consider whether the extreme youth of the low-metallicity galaxies is responsible for the lack of PAH emission.