Mid-infrared characteristics of star-forming complexes in irregular galaxies

We present Spitzer Space Telescope Infrared Spectrograph 10-37 mu m high-resolution spectral snaps of 11 starforming regions in five irregular galaxies. We measure or place upper limits on nebular fine-structure lines, molecular lines from warm H-2, and various polycyclic aromatic hydrocarbon (PAH) features in order to probe the hardness of the stellar radiation field, the electron density of the nebulae, and the physical environment in which PAHs are being heated. The nebulae have similar excitations, corresponding to a midtype O star, and electron densities are low in most regions. PAHs were detected in all but two nebulae, and PAH emission relative to stars or Ha emission tends to be higher in galaxies with higher oxygen abundances. The population of PAHs appears to be different in Im galaxies than it is in spirals. We model emission from warm H-2 in one nebula to obtain a temperature of 800(-340)(+440) K, a neutral gas density >= 10(5) cm(-3), and a far-UV radiation field of G(o) >= 10(3). Variations in excitation across the nebulae are small, but the PAH 11.2 mu m emission varies by factors of 3-4 across the maps. Differences in the local H I or H II environment of star-forming regions result in only mild differences in mid-IR ratios of nebular fine-structure line emission among In galaxies. Variations in PAH emission, however, are snore evident.