ISO-SWS spectra of galaxies: Continuum and features

We present an inventory of mid-infrared spectral features detected in high resolution (R similar to 1500) ISO-SWS 2.4-45 mu m spectra of the galaxies M 82, NGC 253, Circinus, NGC 1068, and a position in the 30 Doradus region of the Large Magellanic Cloud. We discuss their identifications and highlig ht possible relations between these features and the physical state of the interstellar medium in galaxies. The spectral features vary considerably fr om source to source in presence and relative strength. Emission features ar e largely absent in the intense radiation field close to an AGN. Compared t o normal infrared-selected starbursts, they also seem to be weaker in a low metallicity, intensely star forming environment. The large number of featu res beyond 13 mu m is remarkable. Some of the features have - to our knowle dge - not been reported before in astronomical objects. In the 5-13 mu m region, emission from unidentified infrared bands (UIBs), usually ascribed to aromatic molecules, and apparent silicate absorption do minate the spectrum. The density of features makes it difficult to determin e the continuum, particularly in ground-based data of limited wavelength co verage. In fact the apparent depth of the 9.7 mu m silicate absorption may be overestimated in the presence of UIB emission, as we demonstrate by comp aring the spectrum of M 82 to the (absorption free) spectrum of the reflect ion nebula NGC 7023. No strong silicate absorption is present in M 82. The (very small grain) dust continuum under the UIB emission in our starburst t emplates can be modeled by a simple power law, starting at wavelengths betw een 8 and 9 mu m. We find broad H2O-ice absorption features at 3.0 mu m in M 82 and NGC 253. Their optical depths (relative to the visual extinction) indicate that the lines of sight towards these galaxies have similar properties as the line o f sight towards the Galactic Center. The active galaxy NGC 1068 exhibits a clearly different spectrum of absorption features, indicating different phy sical conditions in the obscuring regions of this AGN compared to the starb urst templates. The spectra are valuable templates for future mid-infrared missions. We smo oth our data to simulate low resolution spectra as obtained with ISOCAM-CVF ISOPHOT-S, and in the future with the low resolution mode of SIRTF-IRS, an d use our high spectral resolution information to highlight possible identi fication problems at low resolving power that are caused by coincidences of lines and features. The spectra are available in electronic form from the authors.