Ionization structure of the halo of ESO 338. Panel a: ionization parameter map of the central area of ESO 338 as traced by the [O III]λ 5007 Å/[S II] λ(6717 + 6731) Å line ratio. The blue contours show the He II λ4686 Å emission. The He II image was smoothed with 3 pixel Gaussian kernel. The contour levels range from 50 to 1300 × 10⁻²⁰ erg s⁻¹ cm⁻². Panel b: ionization map as derived from the [O III]5007 Å over Hα line ratio. The black contours are the I-band contours similar to Fig. 4. The white box denotes the borders of the slightly zoomed in [O III]/[S II] ionization map in panel a and the extinction map in Fig. 5.