Hubble Space Telescope Imaging of the Extremely Metal-Poor Galaxy SDSS J0133+1342

We present deep U, narrow V and I-band images of the extremely metal-poor (12 + log(O/H) = 7.60) galaxy SDSSJ0133+1342 obtained with the Advanced Camera for Surveys of the Hubble Space Telescope. The images reveal the galaxy to have an irregular morphology and a maximum size of only ∼ 400 pc. However, it contains at least 17 globular clusters with a large range in color and luminosity, which together contribute approximately 15% of the galaxy's luminosity. The bluest and most luminous of these clusters have L > 10⁶ L_sun and qualify as super star clusters. Using single-burst stellar population synthesis models of the appropriate metallicity, we find that the colors of the reddest clusters are best matched by stars ∼ 10 Gyr old; their colors cannot be produced by the reddening of younger and bluer stars for plausible amounts of dust. These results indicate that SDSS J0133+1342 has a complicated formation history, possibly as the result of a series of collisions between two or more small clumps of stars as they coalesce and eventually form a quiescent dwarf galaxy. The presence of evolved stars in this object indicates that, in contrast to recent results for I Zw 18, not all extremely metal-poor dwarf galaxies in the local universe are forming their first generation of stars. This work was supported by NASA through grant HST-GO-10180.06-A to the Space Telescope Science Institute.