For a long time I Zw18 seemed to play in its own league, with no other BCG coming really close to its low oxygen abundance in the H II-gas. However the entrance of SBS0335-052 on the stage changed the situation. This galaxy was found in the Second Byurakan Survey (SBS, Markarian and Stepanian 1983). A number of papers from 1990 and onwards have shown it to be a galaxy with an oxygen abundance comparable to that of IZw18 (Izotov et al. 1990, 1997b; Melnick et al. 1992). Melnick et al. (1992) and Izotov et al. (1997b) both find an oxygen abundance of 1/40 of the solar value. The analysis by Melnick et al. (1992) suggest that the O abundance may be a factor of two higher in a north-western H II region. Thuan et al. (1997) argues that the oxygen abundance in the neutral gas may be even smaller by a factor 100. Mid infrared observations with ISO revals the presence of dust, and a gas to dust ratio typical for more metal rich BCGs (Thuan et al. 1999). VLA observations have revealed an H I mass of ~ 2 x 109 M and a dynamical mass a factor of a few larger (Pustilnik et al. 1999). An image of SBS0335-052 from the HST archive (see also Thuan et al. 1997) is shown in Fig. 9 and reveals a complex morphology. Like IZw18, SBS0335-052 has a faint companion galaxy (Sect. 5.2.1).
Figure 9. SBS0335-052 imaged with the Hubble Space Telescope in the V-band, cf. Thuan et al. (1997). The size of the image is 20 x 20 arcseconds corresponding to 5.2 x 5.2 kpc. Note the extended complex filaments. (Obtained from the HST archive.)
There have been several claims that this galaxy is a truly young galaxy, not containing any underlying old population, (Thuan et al. 1997, Izotov et al. 1997b, Papaderos et al. 1998). The argument put forward is the low metallicity and the lack of any underlying population in surface photometric data. However Lipovetsky et al. (1999) in their study of the companion, present also surface photometry of SBS0335-052. The R - I colours at a radius of 10 arcseconds is (R - I) = 0.4 which indicates an age of several Gyr for a single stellar population model with metallicity 2% solar and a standard Salpeter IMF (Bruzual and Charlot 2000). Nebular gas contamination cannot account for this, since that would make R - I bluer, leading to an underestimate of the age. Even allowing for an internal extinction of E (B - V) = 0.2 an age of 1 Gyr or more is required. Changing IMF, or metallicity (to 20% solar) still leads to an age in excess of 1 Gyr. Relaxing the assumption of instantaneous star formation, and allowing for more realistic SFHs would increase the age to around 10 Gyr (for = 3 Gyr, cf. Sect. 5.1.2). Thus, in view of the surface photometry presented by Lipovetsky et al. (1999), the age of > 1 Gyr can be regarded as a conservative lower limit. Moreover the presence of red star clusters (Östlin 1999b) suggests an age of several Gyr. These star clusters were discussed by Papaderos et al. (1998) but the models they used to interpret the ages do not agree with other published models, and are apparently erroneous leading to an underestimate of the age of the clusters. Thus, SBS0335-052, like IZw18, is probably not a genuinely young galaxy. In some sense SBS0335-052 is more extreme than IZw18 in that despite being almost as metal-poor as IZw18, it is more luminous (MB = -16.7) and thus lies further off the metallicity-luminosity relation for dIs, see Fig. 10.