Herschel/PACS map of [C ii] (top), [O i] (middle), and [O iii] (bottom) emission in I Zw 18. The 25 spaxels of the PACS footprint are overplotted on the F555W HST/ACS image. The contours show the H i column density at 5″ resolution (Lelli et al. 2012). For display purposes, only the fits are shown for each spaxel with detection level >2σ, and the number indicates the detection level in σ. The red circle shows the beam size and the red cross shows the emission centroid as calculated by the optimal extraction (Sect. 3.1.2). Individual spaxel spectra are presented in Appendix A.1.