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 Infrared Array Camera and Multiband Imaging Photometer for Spitzer 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 Infrared Spectrograph (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: [O IV] emission is {approx}>4 times as common as [Fe II] emission. They also have a more intense ISRF, as indicated by the 71 to 160 {mu}m luminosity ratio. Two-thirds of the sample (15 BCDs) show polycyclic aromatic hydrocarbon (PAH) features, although the fraction of PAH emission normalized to the total infrared (IR) luminosity is considerably smaller in metal-poor BCDs ({approx}0.5%) than in metal-rich star-forming galaxies ({approx}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 H{sub 2} rotational lines, and more than a third of the objects in our sample (eight BCDs) have {approx}>3sigma detections in one or more of the four lowest-order transitions. The warm gas masses in the BCDs range from 10{sup 3} M{sub sun} to 10{sup 8} M{sub sun}, and can be comparable to the neutral hydrogen gas mass; relative to their total IR luminosities, some BCDs contain more H{sub 2} than Spitzer Nearby Galaxy Survey galaxies.