The mid-infrared properties of starburst galaxies from Spitzer-IRS spectroscopy

We present 5-38 mu m mid-infrared spectra at a spectral resolution of R approximate to 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 tau(9.8 mu m) similar to 5. The spectral slopes can be used to discriminate between starburst and AGN powered sources. The monochromatic continuum fluxes at 15 and 30 mu m 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-IR as both continuum and PAH feature scale proportionally. However, the luminosity of the 6.2 mu m feature scales with L-IR and 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 II]/[Ne II] ratio, we found no systematic correlation between the radiation field hardness and the PAH equivalent width or the 7.7 mu m/11.3 mu m 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.