The metal deficient ( Z = Z _ { \odot } /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 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 grey-body spectrum , modified by an extinction law similar to that observed toward the Galactic Center , with an optical depth of A _ { V } \sim 19-21 mag .
Such a large optical depth implies that a large fraction ( as much as \sim 75 % ) of the current star-formation activity in SBS 0335-052 is hidden by dust with a mass between 3 \times 10 ^ { 3 } M _ { \odot } and 5 \times 10 ^ { 5 } M _ { \odot } .
Silicate grains 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 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 .