Optical & NIR surface photometry of I Zw 18 ∗

Using HST and ground-based optical and NIR data, we investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 has an extended low-surface-brightness (LSB) older stellar popu- lation underlying the star-forming regions, as is the case in evolved iE/nE BCDs. Subtraction of narrow band Hand (O III) exposures from R and V images shows that the filamentary LSB envelope extending out to ∼2 kpc away from the starburst region, and hence the optical broad-band colors observed therein, are due mainly to ionized gas emission. Ionized gas accounts already at a galactocentric distance of 0.7 kpc for more than 80% of the R band line-of-sight intensity and contributes more than 40% of the integrated R band light of I Zw 18. The struc- tural properties (such as the exponential scale length) of the stellar LSB component underlying the extended ionized gas emission place I Zw 18 among the most compact BCDs studied so far. Contrary to evolved nE/iE BCDs the stellar component in I Zw 18 shows no appreciable color gradients over a range of ∼8 mag in surface brightness. In order to understand the dynamical formation and the evolutionary state of I Zw 18, it is impor- tant to investigate whether or not the star-formation activity in this system is taking place on top of an extended and evolved stellar low-surface-brightness (LSB) host. If the putative underlying older stellar population has photometric properties typical of evolved (a few Gyr old) iE/nE BCDs (cf. Loose & Thuan 1985), then its detection should be feasible with current instrumentation. The underlying LSB component of BCDs shows for MB > -16 mag a central surface brightness excess of >1.5 mag and an exponential scale length reduced by a factor of ∼2 as compared to, e.g., dwarf irregulars (Papaderos et al. 1996; hereafter P96, Patterson & Thuan 1996, Salzer & Norton 1999), i.e. its mean surface brightness is significantly higher than in other dwarf galaxy types. More- over, in such systems the emission of the evolved stellar host contributes on average about half of the B band light within the 25 B mag/⊓ '' isophote and dominates the radial intensity and color