5 The PAH Emission Deficit In Low - Metallicity Galaxies - A Spitzer View

Archival observations of 18 starburst galaxies that span a wide range in metal-licity reveal for the first time a correlation between the ratio of emission line fluxes of [FeII] at 26 µm and [NeII] at 12.8 µm and the 7.7 µm PAH strength, with the [FeII]/[NeII] flux ratio decreasing with increasing PAH strength. We also find a strong correlation between the [FeII]/[NeII] flux ratio and the host galaxy metallicity, with the flux ratio decreasing with increasing metallicity. Since [FeII] emission has been linked primarily to supernova shocks, we attribute the high [FeII]/[NeII] ratios in low-metallicity galaxies to enhanced super-nova 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.