Dust in an extremely metal-poor galaxy: Mid-infrared observations of SBS 0335-052

The metal deficient (Z = Z(circle dot)/41) Blue Compact Dwarf Galaxy (BCD) SBS 0335-052 was observed with ISOCAM between 5 and 17 mu m. With a L-12 mu m/L-B ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared. The spectral energy distribution (SED) is dominated by a very strong continuum which makes the ionic lines of [SIV] and [NeIII] very weak. From 5 to 17 mu m the SED can be fitted with a grey-body spectrum, modified by an extinction law similar to that observed toward the Galactic Center, with an optical depth of A(V) similar to 19-21 mag. Such a large optical depth implies that a large fraction (as much as similar to 75%) of the current star-formation activity in SBS 0335-052 is hidden by dust with a mass between 3x10(3)M(circle dot) and 5x10(5)M(circle dot). It is remarkable that such a nearly primordial environment contains as much dust as galaxies which are 10 times more metal-rich. If the hidden star formation in SBS 0335-052 is typical of young galaxies at high redshifts, then the cosmic star formation rate derived from UV/optical fluxes would be underestimated.