A mid-infrared spectroscopic survey of starburst galaxies: Excitation and abundances

We present spectroscopy of mid-infrared emission lines in twelve starburst regions, located in eleven starburst galaxies, for which a significant number of lines between 2.38 and 45 mum were observed with the ISO Short Wavelength Spectrometer, with the intention of providing a reference resource for mid-infrared spectra of starburst galaxies. The observation apertures were centred on actively star forming regions, including those which are inaccessible at optical wavelengths due to high levels of obscuration. We use this data set, which includes fine structure and hydrogen recombination lines, to investigate excitation and to derive gas phase abundances of neon, argon, and sulphur of the starburst galaxies. The derived Ne abundances span approximately an order of magnitude, up to values of similar to3 times solar. The excitation ratios measured from the Ne and Ar lines correlate well with each other (positively) and with abundances (negatively). Both in excitation and abundance, a separation of objects with visible Wolf-Rayet features ( high excitation, low abundance) is noted from those without (low excitation, high abundance). For a given abundance, the starbursts are of relatively lower excitation than a comparative sample of H II regions, possibly due to ageing stellar populations. By considering the abundance ratios of S with Ne and Ar we find that, in our higher metallicity systems, S is relatively underabundant by a factor of similar to3. We discuss the origin of this deficit and favour depletion of S onto dust grains as a likely explanation. This weakness of the mid-infrared fine structure lines of sulphur has ramifications for future infrared missions such as SIRTF and Herschel since it indicates that the S lines are less favourable tracers of star formation than is suggested by nebular models which do not consider this effect. In a related paper (Sturmetal. 2002), we combine our results with spectra of Seyfert galaxies in order to derive diagnostic diagrams which can effectively discriminate between the two types of activity in obscured regions on the basis of excitation derived from detected mid-infrared lines.