Three-color composite image of I Zw 18 and I Zw 18 C, combining HST ACS data in V, R, and I (blue, green, and red channels, respectively). The position of the SF regions NW and SE is indicated by crosses. The regions labeled “Loop” and “Hα arc” were studied through deep Keck II long slit spectroscopy by Izotov et al. (2001a). The region ω at the southeastern tip of I Zw 18 shows comparatively weak nebular emission, its colors therefore allow meaningful constraints to be placed on the age of the stellar component (Papaderos et al. 2002). In the magnified version of region NW (inset), combining the unsharp masked images I_c, R_c, and V_c (see Sect. 3 for details), about 30 point sources, surrounded by a complex network of ionized gas shells are discernible. The irregular blue galaxy I Zw 18 C is located  ~22′′ northwest of region NW ( ≈ 2 kpc at the assumed distance of 19 Mpc to I Zw 18). It shows faint nebular emission in its bluer southeastern tip and central star cluster complex C (Izotov et al. 2001a). North is at the top and east to the left.