Abstract
I Zw 18, a blue compact dwarf (BCD) galaxy with the second lowest metallicity measured in a star-forming object, has been observed with all three instruments on board the Spitzer Space Telescope. We present the deepest 5-36 μm mid-infrared (mid-IR) spectrum of this galaxy as yet obtained, as well as 3.6-70 μm imaging results. As with SBS 0335-052E, another BCD with similar metallicity, I Zw 18 shows no detectable emission from polycyclic aromatic hydrocarbons (PAHs). However, the continuum emission, from 15 to 70 μm, of I Zw 18 has a much steeper slope, more characteristic of a typical starburst galaxy of solar abundance. The neon abundance as measured from the infrared fine-structure lines is ~1/23 Z☉, and the sulfur abundance is ~1/35 Z☉, generally consistent with the nebular oxygen abundance of 1/30 Z☉ derived from optical lines. This suggests that the extinction to the infrared-emitting regions of this galaxy is low, also in agreement with the optical Balmer line ratios.
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