METALLICITY EFFECTS ON MID-INFRARED COLORS AND THE 8μm PAH EMISSION IN GALAXIES

We examine colors from 3.6μm to 24μm as a function of metallicity (O/H) for a sample of 34 galaxies. The galaxies range over 2 orders of magnitude in metallicity. They display an abrupt shift in the 8μm to 24μm color between metallicities 1/3 to 1/5 of the solar value. The mean 8μm to 24μm flux density ratio below and above 12 + log(O/H) = 8.2 is 0.08 ± 0.04 and 0.70 ± 0.53, respectively. We use mid-infrared colors and spectroscopy to demonstrate that the shift is primarily due to a decrease in the 8μm flux density as opposed to an increase in the 24μm flux density. This result is most simply interpreted as due to a weakening at low metallicity of the mid-infrared emission bands usually attributed to PAHs (polycyclic aromatic hydrocarbons) relative to the small-grain dust emission. However, existing empirical spectral energy distribution models cannot account for the observed short-wavelength (i.e., below 8μm) colors of the low-metallicity galaxies merely by reducing the strength of the PAH features; some other emission source (e.g., hot dust) is required. Subject headings: galaxies: ISM—infrared: galaxies