The deeply embedded starburst in SBS 0335-052

We present 4-micron ISAAC imaging and spectroscopy of the extremely metal-poor dwarf galaxy SBS 0335-052, aimed at a better understanding of the dust in this low-metallicity galaxy. The 4-micron emission turns out to be very compact, confined to the brightest pair of Super Star Clusters (SSCs). The Ks-L$^\prime$ color is extremely red, and the $L^\prime$ emission is consistent with the extrapolation of the ISO mid-infrared spectral energy distribution (SED). From hydrogen recombination lines and a fit to the near-/mid-infrared SED, we confirm a visual extinction of >= 15 mag. Our data suggest that the sites of the optical and infrared emission are distinct: the optical spectral lines come from an almost dustless region with a high star formation rate and a few thousand OB stars. This region lies along the line-of-sight to a very dusty central star cluster in which there are more than three times as many massive stars, completely hidden in the optical. From the extinction, we derive an upper limit for the dust mass of 10^5 M$_\odot$ which could be produced by recent supernovae.