ISOLWS Spectroscopy of M82: A Unified Evolutionary Model

We present the first complete far-infrared spectrum (43 to 197 um) of M82,the brightest infrared galaxy in the sky, taken with the Long WavelengthSpectrometer of the Infrared Space Observatory (ISO). We detected seven finestructure emission lines, [OI] 63 and 145 um, [OIII] 52 and 88 um, [NII] 122um, [NIII] 57 um and [CII] 158 um, and fit their ratios to a combinationstarburst and photo-dissociation region (PDR) model. The best fit is obtainedwith 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, withour best fit being a 3-5 Myr old instantaneous burst model with a 100 M_ocut-off. We also detected the ground state rotational line of OH in absorptionat 119.4 um. No excited level OH transitions are apparent, indicating that theOH is almost entirely in its ground state with a column density ~ 4x10^{14}cm^{-2}. The spectral energy distribution over the LWS wavelength range is wellfit with a 48 K dust temperature and an optical depth, tau_{Dust} proportionalto lambda^{-1}.Comment: 23 pages, 4 figures, accepted by ApJ, Feb. 1, 199