The Extraordinary mid - Infrared Spectrum of the Blue Compact Dwarf Galaxy

SBS0335-052 is a blue compact dwarf galaxy (BCD) with one of the lowest known metallicities, Z∼Z⊙/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, emission lines of [SIV] and [NeIII], and puts strong upper limits on the PAH emission features. The observed low resolution spectrum (R∼90) extends from 5.3 to 35μm and peaks at ∼28μm. The spectrum is compared to IRS observations of the prototypical starburst nucleus NGC7714. SBS 0335-052 is quite unlike normal starburst galaxies, which show strong PAH bands, low ionization emission lines, and a continuum peak near 80μm. The continuum difference for λ > 30μm 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 AGNs which also show relatively featureless flat spectra in the mid-IR. Subject headings: dust, extinction — galaxies: individual (SBS 0335-052) — galaxies: starburst