2004A&A...425...51P

We present the results of deep multicolour CCD imaging with the SAO RAS 6m telescope of the pair of extremely metal-deficient gas-rich dwarf galaxies SBS 0335-052 E and W. The total magnitudes in U,B,V,R and I bands and the integrated fluxes of Hα emission are measured for both galaxies, and their integrated colours are derived. The analysis of their surface brightness (SB) distributions is performed with the use of the azimuthally-averaged SB profiles. The latter were modeled by the central Gaussian component and the underlying exponential ``disk'', mainly contributing in the outermost, very low SB regions. The colours of these LSB components are used to estimate the age of the oldest visible stellar population. For the interpretation of the observed LSB colours their contamination by the nebular emission of ionized gas is accounted for by the use of the distribution of Hα flux. We compare the derived ``gas-free'' colours with the colours predicted by the evolution synthesis models from the PEGASE.2 package, considering three SF histories: a) instantaneous starburst; b) continuous star formation with constant SFR; and c) continuous exponentially fading SF (e-fold time of 3Gyr). We conclude that the ``gas-free'' colours of the LSB component of the Eastern galaxy can be best consistent with the instantaneous starburst population (ages of ≲100Myr). Models with continuous SF give less consistent results, but can be considered as acceptable. For such scenarios, the ``gas-free'' colours require ages of ≲400Myr. For the Western galaxy, the situation is in general similar. But the colour (V-R) appears to be quite red and implies a significantly older component. We briefly discuss the possible evolution sequence between SBS 0335-052, HI 1225+01 and other extremely metal-deficient galaxies based on the merger scenario.