The extraordinary mid-infrared spectrum of the blue compact dwarf galaxy SBS 0335-052

SBS 0335-052 is a blue compact dwarf galaxy (BCD) with one of the lowest known metallicities, Z similar to Z(circle dot)/41, making it a local example of how primordial starburst galaxies and their precursors might appear. A spectrum obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope clearly shows silicate absorption features and emission lines of [S IV] and [Ne III], and puts strong upper limits on the polycyclic aromatic hydrocarbon (PAH) emission features. The observed low-resolution spectrum (R similar to 90) extends from 5.3 to 35 mum and peaks at similar to28 mum. The spectrum is compared to IRS observations of the prototypical starburst nucleus NGC 7714. SBS 0335-052 is quite unlike normal starburst galaxies, which show strong PAH bands, low-ionization emission lines, and a continuum peak near 80 mum. The continuum difference for lambda > 30 mum implies a substantial reduction in the mass of cold dust. If the spectrum of this very low-metallicity galaxy is representative of star-forming galaxies at higher redshifts, it may be difficult to distinguish them from active galactic nuclei, which also show relatively featureless flat spectra in the mid-IR.