Since its launch in November 1995, the Infrared Space Observatory ISO has converted mid-infrared spectroscopy into a mature tool. Due to its proximity, brightness, and the wealth of complementary information, the center of our Galaxy remains a unique testbed for studies of galactic nuclei. A first analysis of the 2.4-45μm spectrum obtained with ISO-SWS (Fig. 1) has been presented by Lutz et al. (1996a). One of the surprises is a determination of the mid-infrared extinction law from comparison of the Galactic center hydrogen recombination line fluxes and case B expectations: The extinction law clearly lacks the deep minimum near 7μm expected for standard graphite/silicate mixes. External galaxies are less close to the ideal ‘screen’ case and cannot provide the detail of the GC recombination line spectrum needed to derive an extinction law. However, simple ratios of the strongest recombination lines are inconsistent with a classical extinction law and any dust configuration.
Two New Recombination-Line Results