A MIR Spectroscopic Survey of Starburst Galaxies

We present a mid-infrared (MIR) spectroscopic survey of starburst galaxies as an example of analysis of scientifically focused samples selected from the ISO Data Archive (IDA). We use fine structure lines (FSL) ratios of Ne, Ar and S to construct diagnostic excitation diagrams and, in combination with hydrogen recombination lines (HRL), we determine their elemental abundances. For Ne and Ar, we find that excitation indicators are positively correlated with each other and negatively with abundance. On comparison with a complementary sample of galactic H II regions we find that starbursts are generally of lower excitation. Starbursts exhibiting Wolf-Rayet (WR) features are separated both in excitation and abundance from the remaining starbursts. Most surprisingly, S is found to be underabundant by a factor of ∼ 3 in our low excitation starbursts with respect to the Ne and Ar, contrary to expectations for nucleosynthesis theory. Our results are combined with those of a related sub-sample of Seyfert galaxies (Sturm et al. 2002) to derive diagnostic diagrams discriminating the two types of activity on the basis of excitation traced by MIR lines. The data presented will be useful as a reference for observations of fainter and/or higher redshift sources with future IR observatories such as SIRTF, SOFIA and Herschel.