2006ApJ...653.1129B

We present 5-38 µm mid-infrared spectra at a spectral resolution of R~65-130 of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The spectra show a vast range of starburst SEDs. The silicate absorption ranges from essentially no absorption to heavily obscured systems with an optical depth of τ_9.8µm∼5. The spectral slopes can be used to discriminate between starburst and AGN powered sources. The monochromatic continuum fluxes at 15 and 30 µm enable a remarkably accurate estimate of the total infrared luminosity of the starburst. We find that the PAH equivalent width is independent of the total starburst luminosity L_IRas both continuum and PAH feature scale proportionally. However, the luminosity of the 6.2 µm feature scales with L_IRand can be used to approximate the total infrared luminosity of the starburst. Although our starburst sample covers about a factor of 10 difference in the [Ne III]/[Ne II] ratio, we found no systematic correlation between the radiation field hardness and the PAH equivalent width or the 7.7 µm/11.3 µm PAH ratio. These results are based on spatially integrated diagnostics over an entire starburst region, and local variations may be ``averaged out.'' It is presumably due to this effect that unresolved starburst nuclei with significantly different global properties appear spectrally as rather similar members of one class of objects.