ISO LWS Spectroscopy of M82: A Unified Evolutionary Model

February 1999 • 1999ApJ...511..721C

Authors • Colbert, James W. • Malkan, Matthew A. • Clegg, Peter E. • Cox, Pierre • Fischer, Jacqueline • Lord, Steven D. • Luhman, Michael • Satyapal, Shobita • Smith, Howard A. • Spinoglio, Luigi • Stacey, Gordon • Unger, Sarah J.

Abstract • We present the first complete far-infrared spectrum (43-197 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 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 G₀=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_solar cutoff. 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 entirely in its ground state with a column density ~4×10¹⁴ 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, τ_Dust ~ λ^-1.

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James Colbert

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