Dust in the Extremely Metal-Poor Blue Compact Dwarf Galaxy IZw18: The Spitzer Mid-Infrared View

Abstract

IZw18, a blue compact dwarf (BCD) galaxy with the 2nd 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 micron mid-infrared (mid-IR) spectrum of this galaxy as yet obtained, as well as 3.6 to 70 micron imaging results. As with SBS0335-052E, another BCD with similar metallicity, IZw18 shows no detectable emission from polycyclic aromatic hydrocarbons (PAHs). However, the continuum emission, from 15 to 70 micron, of IZw18 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_sun, and the sulfur abundance is ~ 1/35 Z_sun, generally consistent with the nebular oxygen abundance of 1/30 Z_sun 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.