Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved

1. INTRODUCTION: From IRAS to ISO

In 1983 the first cryogenic infrared astronomy satellite, IRAS ², surveyed 96% of the sky in four broad-band filters at 12, 25, 60, and 100 µm, to limits 1 Jy (Neugebauer et al 1984, see reviews of Beichman 1987, Soifer et al 1987). IRAS detected infrared (IR) emission from about 25,000 galaxies, primarily from spirals, but also from quasars (QSOs) (Neugebauer et al 1986, Sanders et al 1989), Seyfert galaxies (de Grijp et al 1985) and early type galaxies (Knapp et al 1989, 1992). IRAS discovered a new class of galaxies that radiate most of their energy in the infrared (Soifer et al 1984), many of them dusty starburst galaxies ³. The most luminous of these infrared galaxies [(ultra-)luminous infrared galaxies: (U)LIRGs or (U)LIGs] have QSO-like bolometric luminosities (LIRGs: L 10¹¹ L, ULIRGs: L 10¹² L; Sanders & Mirabel 1996).

The Infrared Space Observatory (Kessler et al 1996) was the first cryogenic space infrared observatory. ISO was launched in November 1995. It was equipped with a multi-pixel near-IR/mid-IR camera (ISOCAM; Cesarsky et al 1996, Cesarsky 1999), a multiband mid- and far-IR spectro-photometer (ISOPHOT; Lemke et al 1996, Lemke & Klaas 1999), a 2.4-45 µm spectrometer (SWS; de Graauw et al 1996, de Graauw 1999) and a 43-197 µm spectrometer (LWS; Clegg et al 1996, Clegg 1999). The ISO mission lasted until April 1998, about one year longer than expected (Kessler 1999). The present review is an account of the key extragalactic results of ISO as of December 1999. We also refer the reader to the special Astronomy and Astrophysics issue on early ISO results (volume 315, No. 2, 1996), to the proceedings of the 1998 Paris conference (The Universe as Seen by ISO; Cox & Kessler 1999), and to the recent review by Cesarsky & Sauvage (2000).

² The Infra-Red Astronomical Satellite was developed and operated by the US National Aeronautics and Space Administration ( NASA), the Netherlands Agency for Aerospace Programs (NIVP) and the United Kingdom Science and Engineering Research Council (SERC). Back.

³ Following the classical analysis by Rieke et al (1980) of two nearby representatives of this class, M82 and NGC 253, these galaxies are presently going through a very active, but short-lived 'starburst' of duration of a few tens of millions of years or less. For a 'Salpeter' initial mass function (IMF) from 100 to 1 M a luminosity of 10¹⁰ L corresponds to a star formation rate of roughly 1 M yr^-1 (e.g. Kennicutt 1998). Given its infrared luminosity ( 4 × 10¹⁰ L) and central gas content (2.5 × 10⁸ M), the relatively small galaxy M82 thus cannot sustain its present star formation rate for much longer than 50 million years, hence the term 'starburst'. Back.