HST/ACS image of I Zw 18 (main body), with RBG colors defined as I, V, and B filters. Images were downloaded from the Hubble Legacy Archive (http://hla.stsci.edu/) and later combined. The green contours show the H i column density distribution from Lelli et al. (2012) with 2′′ resolution (beam size in the bottom left). Contours are drawn for 3 (dashed), 6, 9, 12, and 15 × 10²¹ cm^-2. The COS observation, represented by the black circle (2.5″ diameter), is centered on the NW star cluster.