ISO's telescope and instruments are surrounded by a large tank of superfluid helium cooling them to a few degrees above absolute zero. At this temperature, the disturbing infrared emission from the telescope itself is completely suppressed. For years, astronomers have hotly debated how this could trigger the huge luminosity. Others thought that, like in distant quasars, a central black hole is 'fed' with large amounts of matter, causing it to shine unusually bright. The major difficulty in solving this puzzle are the huge amounts of dust hiding the centers of these galaxies from our direct view.
Dust absorbs light and re-emits it as infrared radiation.
Thereby dust is the origin of the huge infrared luminosity, but it also prevents us from seeing the centers of these galaxies in visible light. All that could be observed until recently is a thin layer at the surface of the galaxy we see in visible light, and the infrared reradiation by warm dust, which however tells very little about how the dust is actually heated. With the ISO-SWS, it is now possible to observe at wavelengths where the absorption is much less severe, penetrating the dust clouds enshrouding the centers of galaxies, and to look for characteristic signatures of the ultimate energy source. Here, it is crucial that SWS can obtain spectra, that means give precise wavelength information for the detected radiation. Among the spectral lines detected for the first time are those of molecular hydrogen, which reveal that the fierce radiation heats much of the dense molecular gas to surprisingly high temperatures.