ISO LWS spectroscopy of M82:: A unified evolutionary model

We present the first complete far-infrared spectrum (43-197 mu m) 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 mu m, [O III] 52 and 88 mu m, [N II] 122 mu m, [N III] 57 mu m, and [C II] 158 mu m, and fitted their ratios to a combination starburst and photodissociation region (PDR) model. The best fit is obtained with H II regions with n = 250 cm(-3), an ionization parameter of 10(-3.5), and PDRs with n = 10(3.3) cm(-3) and a far-ultraviolet flux of G(0) = 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-. cutoff. We also detected the ground-state rotational line of OH in absorption at 119.4 mu m. No excited level OH transitions are apparent, indicating that the OH is almost entirely in its ground state with a column density similar to 4 x 10(14) cm(-2). The spectral energy distribution over the long-wavelength spectrometer wavelength range is well fitted with a 48 K dust temperature and an optical depth, tau(Dust) proportional to lambda(-1).