The Extraordinary Mid‐infrared Spectrum of the Blue Compact Dwarf Galaxy SBS 0335−052

Abstract

SBS 0335-052 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 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 ∼ 90) extends from 5.3 to 35 μm and peaks at ∼28 μm. 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 μm. The continuum difference for λ > 30 μm 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 re dshifts, it may be difficult to distinguish them from active galactic nuclei, which also show relatively featureless flat spectra in the mid-IR.