Relation between the IR8 observed in stacked Spitzer and Herschel photometry, and the gas-phase metallicity. The metallicity, expressed in terms of oxygen abundance 12 + log ₁₀(O/H) (the solar value is 8.73, Asplund et al. 2009), was estimated using the Fundamental Metallicity Relation (FMR, Mannucci et al. 2010). Data points are colored with redshift as indicated in the legend. We also show the galaxies from the HRS as gray circles, using their measured metallicities. The black line shows the best fitting power law to the stacked data. The z = 0 relation obtained by Galliano et al. (2008) is shown for reference with a dotted gray line, converting their measured PAH mass fractions to IR8 using Eq. (14).