Based on observations with the WFI instrument at 2.2-m ESO/MPI telescope and ESO NTT.
In collaboration with A.Y. Kniazev (ESO and SAAO) and L. Vanzi (ESO)

Problem of ages of XMD galaxies is addressed through the comparison of colours of stars in their outer regions (where one can neglect the effect of recent star-formation activity), with model tracks of evolving stellar populations from, e.g., the popular package PEGASE.2 (Fioc & Rocca-Volmerange 1999). For some of XMD galaxies, the contribution of nebular emission in the light of outer regions is negligible or invisible. For them, the measured colours of periphery low surface brightness regions can give directly the estimates of their ages. Here such XMD galaxies - HS 0122+0743 and UM 133 - are presented in the bottom panel. Their BVR colours with +-1-sigma bars are shown overlaid on the two PEGASE.2 model tracks (instantaneous SF episode - red, and continuous SF with the constant star formation rate - black). The accepted metallcity is Z=Zo/10, close to that of the shown galaxies, and the IMF is the classical Salpeter IMF. The numbers along the tracks denote respective ages. The colours of the outer regions in these galaxies are consistent with the continuous SF model at T=5-12 Gyr, that is they are old galaxies.

For other XMD galaxies, the nebular emission makes a significant contribution everywhere (as seen, e.g., from images in H-alpha), so one needs to account for its effect on the colours of the outer regions. The example of application of the procedure, taking nebular emission into account, is shown in the top panel for the XMD galaxy HS 2134+0400, with 12+log(O/H)=7.44. Here the red and black lines show the same PEGASE.2 model tracks for evolving stellar populations. Two squares with the error bars show the positions of BVR colours for the center and periphery of HS 2134+0400. Two squares without bars in the lower left corner show the colours of nebular emission (gas). Red dots correspond to the colours of mixtures of stars and gas (nebular emision) in various proportions. The lower sequence incorporates the stars from the instantaneous starburst with T=4 Myr (as evidenced by the spectrum of the central knot), while the upper sequence combines the emission of gas and stellar population with T=1 Gyr for continuous SF law. The observed colours in the center and at periphery reasonably fit these mixtures. This data suggest that, accounting for the error bars, the colours of visible stars in the outer regions of HS 2134+0400 are consistent with the age of T=1.0+-0.5 Gyr. That is this galaxy can be as young as I Zw 18 and SBS 0335-052.

3 February 2007