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We present the first complete far-infrared spectrum (43 to 197 µm) of M82, the brightest infrared galaxy in the sky, taken with the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO). We detected seven fine structure emission lines, [O i] 63 and 145 µm, [O iii] 52 and 88 µm, [N ii] 122 µm, [N iii] 57 µm and [C ii] 158 µm, and fit their ratios to a combination starburst and photo-dissociation region (PDR) model. The best fit is obtained with HII regions with n = 250 cm −3 and an ionization parameter of 10 −3.5 and PDRs with n = 10 3.3 cm −3 and a far-ultraviolet flux of G o = 10 2.8 . We applied both continuous and instantaneous starburst models, with our best fit being a 3-5 Myr old instantaneous burst model with a 100 M ⊙ cut-off. We also detected the ground state rotational line of OH in absorption at 119.4 µm. No excited level OH transitions are apparent, indicating that the OH is almost -2entirely in its ground state with a column density ∼4x10 14 cm −2 . The spectral energy distribution over the LWS wavelength range is well fit with a 48 K dust temperature and an optical depth, τ Dust ∝ λ −1 .

Abstract. The Long-Wavelength Spectrometer (LWS) is one of two complementary spectrometers aboard the European Space Agency's Infrared Space Observatory 1 (ISO) (Kessler et al., 1996). It operates over the wavelength range 43 196:9 m at either medium (about 150 to 200) or high (6800 to 9700) spectral resolving power. This Letter describes the instrument and its modes of operation; a companion paper describes its performance and calibration.Send offprint requests to: P.E. Clegg (p.e.clegg@qmw.ac.uk) ? ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

A total of 86 galaxies have been detected at 60 µm in the high galactic latitude portion of the IRAS minisurvey. The surface density of detected galaxies with flux densities greater than 0.5 Jy is 0.25 deg-2 • Virtually all the galaxies detected are spiral galaxies and have an infrared to blue luminosity ratio ranging from 50 to 0.5. For the infrared-selected sample, no obvious correlation exists between infrared excess and color temperature. The infrared flux from 10 to 100 µm contributes approximately 5% of the blue luminosity for galaxies in the magnitude range 14 < mpg< 18 mag. The fraction of interacting galaxies is between one-eighth and one-fourth of the sample.

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