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

The metal-deficient (Z = Z(.)/41) blue compact dwarf galaxy SBS 0335--052 was observed with ISOCAM between 5 and 17 mu m. With an L-12 mu m/L-B 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 [S IV] and [Ne III] very weak. From 5 to 17 mu m, the SED can be fitted with a graybody 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 3 x 10(3) and 5 x 10(5) M-.. Silicate grains that are present as silicate extinction bands at 9.7 and 18 mu m 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 that 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.