Normal galaxy studies with ISO have already yielded major progress beyond the knowledge of the IRAS era, improving dramatically our empirical understanding of mid-infrared spectra, mid-infrared images, the global spectral energy distribution and the far-infrared fine-structure lines. This progress has already led to rethinking plans for other surveys and missions addressing those observables.
ISO data are also re-shaping our physical understanding of galaxies: allowing for a better decomposition of the infrared luminosity from a combination of spectral energy distribution and imaging; providing narrower constraints on density and radiation intensity from analysis of fine-structure lines, surface brightness, and the spectral energy distribution; providing a broader perspective on objects with extreme values of luminosity or star formation rate, based on an understanding of the sequence that lies in-between these extremes. Much remains to be learned from ISO, suggesting several years of productive research on the ISO Science Archive still lie ahead.
SIRTF (Space InfraRed Telescope Facility), scheduled for a December 1, 2001 launch, will build on ISO and tackle the new puzzles with its greater sensitivity, finer spatial resolution and larger fields of view. SOFIA (Stratospheric Observatory for Infrared Astronomy) is expected to fly starting in 2002, bringing to the field greater spatial and spectral resolution and instrumental versatility. FIRST (Far-Infrared and Submillimeter Telescope) will then open up the submillimeter spectrum from space starting in 2007, providing access to the colder material in molecular clouds, so all ISM phases from cold clouds to HII regions and shocked gas can be studied, bringing into focus the full picture of the star formation cycle.
I would like to thank the organizers of the Summer School for the invitation to participate in this exciting encounter, and for their patience when it came to submitting the manuscript. James Lequeux in addition provided help and comments on the manuscript. Danny Dale and Alessandra Contursi helped with customized plots, and a careful reading of the manuscript. This work was carried out at the California Institute of Technology, under funding by the National Aeronautics and Space Administration. It was supported in part by the ISO Data Analysis Funding Program, administered for NASA by the Jet Propulsion Laboratory, California Institute of Technology.