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The Spitzer View of Low-Metallicity Star Formation: III. Fine Structure Lines, Aromatic Features, and Molecules

We present low- and high-resolution Spitzer/IRS spectra, supplemented by IRAC and MIPS measurements, of 22 blue compact dwarf (BCD) galaxies. The BCD sample spans a wide range in oxygen abundance [12+Log(O/H) between 7.4 and 8.3], and hardness of the interstellar radiation field (ISRF). The IRS spectra provide us with a rich set of diagnostics to probe the physics of star and dust formation in very low-metallicity environments. We find that metal-poor BCDs have harder ionizing radiation than metal-rich galaxies: [OIV] emission is roughly 4 times as common as [FeII] emission. They also have a more intense ISRF, as indicated by the 71 to 160micron luminosity ratio. Two-thirds of the sample (15 BCDs) show PAH features, although the fraction of PAH emission normalized to the total infrared (IR) luminosity is considerably smaller in metal-poor BCDs (~0.5%) than in metal-rich star-forming galaxies (~10%). We find several lines of evidence for a deficit of small PAH carriers at low metallicity, and attribute this to destruction by a hard, intense ISRF, only indirectly linked to metal abundance. Our IRS spectra reveal a variety of H2 rotational lines, and more than a third of the objects in our sample (8 BCDs) have >=3sigma detections in one or more of the four lowest-order transitions. The warm gas masses in the BCDs range from 10^3 to 10^8 Msun, and can be comparable to the neutral hydrogen gas mass; relative to their total IR luminosities, some BCDs contain more H2 than SINGS galaxies.