Photometric maps of I Zw 18 with Spitzer and Herschel (Rémy-Ruyer et al. 2015). The first panel shows the HST/ACS F555W image for reference, with the NW and SE regions circled in white. The dashed black circle indicates the beam size (from 2′′ for IRAC 8 μm to 12′′ for PACS 160 μm). The 8 μm band is not dominated by PAH emission in I Zw 18 but by stochastically heated small grains and warm grains in thermal equilibrium with the interstellar radiation field.