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 \pm 0.04 and 0.70 \pm 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 can not 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 .