To appear in ApJ Dust in an extremely metal-poor galaxy: mid-infrared observations of SBS

The metal deficient (Z = Z⊙/41) Blue Compact Dwarf Galaxy (BCD) SBS0335-052 was observed with ISOCAM between 5 and 17μm. With a L12μm/LB ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a low-metallicity object. The mid-infrared spectrum shows no sign of the Unidentified Infrared Bands, which we interpret as an effect of the destruction of their carriers by the very high UV energy density in SBS 0335-052. 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 17μm 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 AV ∼ 19-21 mag. Such a large optical depth implies that a large fraction (as much as ∼ 75%) of the current star-formation activity in SBS 0335-052 is hidden by dust with a mass between 3×10 M⊙ and 5×10 5 M⊙. Silicate grains are present as silicate extinction bands at 9.7 and 18 μm can account for the unusual shape of the MIR spectrum of SBS 0335-052. 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 SBS0335-052 is typical of young galaxies at high redshifts, then the cosmic star formation rate derived from UV/optical fluxes would be underestimated. Subject headings: stars: formation – dust, extinction – galaxies: compact – galaxies: individual (SBS 0335-052) – infrared: ISM: continuum – infrared: ISM: lines and bands Based on data obtained with ISO, an ESA project with instruments funded by the ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) with the participation of ISAS and NASA.