SWS observations of the Galactic center

We present a 2.4-45 μm spectrum of the center of our Galaxy obtained with the Short Wavelength Spectrometer (SWS) on board ISO. The wide range of ionic fine structure lines observed yields an average effective temperature for the ionizing stars of 35000 K, with a small contribution of significantly hotter stars, consistent with ageing of an active period of massive star formation that took place a few million years ago. Several absorption features from the foreground cold ISM are detected for the first time. The extinction law towards the Galactic center lacks the expected deep minimum in the 4-8μm range. From the detection of OH 34.6μm absorption, we infer that radiative pumping is likely the major excitation mechanism for OH emission from the Galactic center. We discuss the rich spectrum of iron and nickel and conclude that the [Fe II] spectrum is inconsistent with pure collisional excitation. This calls for caution in interpretation of [Fe II] emission in sources, such as starburst galaxies, which contain intense radiation fields.